The age of the universe: easy question - hard answer

[Ordep]

Hi all,

I've been struggling with an idea lately and was wondering if anybody could shed some light on it. It seems simple but i don't think the answer is very obvious.

The best guestimate for the age of the universe at the moment is 13.5 billion years old. If we could travel back in time 13 billion years and then measured the age of the universe again what would be the answer? Would it be 0.5 billion years old?

Thank you.

 

[mathman]

I am a bit confused as to why you think the obvious answer (i.e. go back 13 bill yrs and it will be .5 bill yrs old) may not be correct?

 

[Garth]

If the value 13.5 Gyr is correct then yes.

Garth

 

[Ordep]

Ok let me expand a little more on where this question comes from. I believe the general way of looking at the universe today is that we live in a sphere of radius 13.5 billion light years with us in the center. This is because in whatever direction one looks from Earth one sees the microwave background radiation (which is a remnant of the big bang) at that distance. I hope you're all nodding your heads in agreement so far.

Now we also know that for every other point in that sphere their perception of the universe is the same (i.e. in every direction they look they'll think that they are in the center of a sphere of radius 13.5 billion light years long.

So my conclusion to this is that when we talk of a different point in that sphere we're basically talking about traveling back in time since everything we see in the heavens is from the past. So to a point located at 1 million light years away from us that point exists 1 million years in the past. Correct?

So this bring me to my question if we were to travel back in time 13 billion years (in other words to a point 13 billion light years away from us) then the age of the universe would still be 13.5 billion years and not 0.5 billion years.

Where is the flaw (if any) in my logic?

Appreciate your comments.

 

[DB]

Ok let me expand a little more on where this question comes from. I believe the general way of looking at the universe today is that we live in a sphere of radius 13.5 billion light years with us in the center.

I like to image a hyperbolic universe, though your bubble (parallel) universe picture is valid and being hypothosised and studied today.

So my conclusion to this is that when we talk of a different point in that sphere we're basically talking about traveling back in time since everything we see in the heavens is from the past. So to a point located at 1 million light years away from us that point exists 1 million years in the past. Correct?

The key about this is that we only see that point (from earth) 1 million light years in the pass, because photons have a finite speed. Traveling to a different point in the universe is not traveling back in time. Though it is possible say your traveling back in time relative to the earth. But your speed will also have an effect of your time perception relative to someone on earth.

So this bring me to my question if we were to travel back in time 13 billion years (in other words to a point 13 billion light years away from us) then the age of the universe would still be 13.5 billion years and not 0.5 billion years.

We don't know how old the universe is, we only are able to view(collect) photons that took 13.5 billion years to reach earth, obviously in all directions, which is why many perceive the universe as a sphere. So saying the universe is 13.5 billion years old is more of a measure of size, then age. We really don't know the age. But it's no less that 13.5 billion years. And remember that the universe is expanding, so turning back time 13 billion years, is like schrinking the universe to "radius" of 0.5 light years. So now it seems a lot easier to reach the end of the universe, I believe its called the final frontier. What lies at th end of the universe...we don't know. Is there an end...?

 

[turbo]

Ok let me expand a little more on where this question comes from. I believe the general way of looking at the universe today is that we live in a sphere of radius 13.5 billion light years with us in the center. This is because in whatever direction one looks from Earth one sees the microwave background radiation (which is a remnant of the big bang) at that distance. I hope you're all nodding your heads in agreement so far.

Basically, that's it in a nutshell.

Now we also know that for every other point in that sphere their perception of the universe is the same (i.e. in every direction they look they'll think that they are in the center of a sphere of radius 13.5 billion light years long.

True. This concept of simultaneity is very important in cosmology. It allows us to visualize the possible extent and geometry of the Universe by extrapolating beyond our visible universe.

So my conclusion to this is that when we talk of a different point in that sphere we're basically talking about traveling back in time since everything we see in the heavens is from the past. So to a point located at 1 million light years away from us that point exists 1 million years in the past. Correct?

Aha! Here's the problem. Above, you are willing to grant that some point near the edge of our visible universe, can have a "now" that is equivalent to ours, and here you equate theoretical travel to that point to "travelling backward in time". This is a relativistic concept that does not coexist well with simultaneity. The location of an observer 13Gly away from us is visible to us due to EM emitted 13Gly ago, but traveling toward him is not equivalent to traveling backward in time. In fact, if you could travel to him at the speed of light and then look around, you would see a universe MUCH older than 26 Billion years old, because in the Big Bang model, the Universe is expanding, and the expansion is accelerating.

So this bring me to my question if we were to travel back in time 13 billion years (in other words to a point 13 billion light years away from us) then the age of the universe would still be 13.5 billion years and not 0.5 billion years.

Where is the flaw (if any) in my logic?

Appreciate your comments.

If you want to travel to the point 13Gly away and view the universe from there as it existed 13Gy ago, you are proposing two separate forms of travel in one trip. One is spatial (go from place A to place B) and the other is temporal (backward along the timeline toward the Big Bang). If you could travel in both these ways, the universe at point B would look only 1/2 billion years old to you. If you want instead to instantly be transported at the present position of place B NOW (in accordance with the simultaneity concept of most cosmologists), the visible universe there would look just like ours in most respects, including an apparent age of 13.5 billion years. Now to confuse things further, (as above) if you want to travel to the present position of observer B at the speed of light (travelling through an expanding Big Bang universe), it will take you MUCH longer than 13 billion years to get there (and if the expansion is accelerating, you will NEVER get there!), and if you get there, the visible universe will be MUCH older than the 26 billion years that you might expect (without figuring in the expansion).

Edit: another PF denizen has answered you while I was composing my lengthy reply (and having supper), but I'm going to let this stand, as a slightly differing point of view.

 

[saltydog]

Hi all,

I've been struggling with an idea lately and was wondering if anybody could shed some light on it. It seems simple but i don't think the answer is very obvious.

The best guestimate for the age of the universe at the moment is 13.5 billion years old. If we could travel back in time 13 billion years and then measured the age of the universe again what would be the answer? Would it be 0.5 billion years old?

Thank you.

Stars in globular clusters surrounding the Milky Way are estimated to be about 15 billion years old.

 

[turbo]

Stars in globular clusters surrounding the Milky Way are estimated to be about 15 billion years old.

And as we discussed in another post, quasars (even at redshifts z~6.5) exist in hosts that exhibit metallicities in the solar and super-solar range. How can an object less than a billion years old have a metallicity equivalent to that of our own galaxy, which apparently has had 13.7Gy of stellar metal-enrichment to produce the chemical properties of our "neighborhood"? If quasars could condense out of materials in our galactic neighborhood, their high metallicities would be non-controversial, but it we believe that heavy elements (heavier than could be produced by BB Nucleosynthesis) are produced by fusion in stars, it is VERY difficult to explain the high metallicities of these early quasars. By this measure alone, the universe is apparently much older than 13.7Gy - MUCH older.

 

[Chronos]

Ok let me expand a little more on where this question comes from. I believe the general way of looking at the universe today is that we live in a sphere of radius 13.5 billion light years with us in the center. This is because in whatever direction one looks from Earth one sees the microwave background radiation (which is a remnant of the big bang) at that distance. I hope you're all nodding your heads in agreement so far.

Actually closer to 13.7 Gly. And this does not have as much to do with the CMB as it does with the Hubble constant - Ho.

Now we also know that for every other point in that sphere their perception of the universe is the same (i.e. in every direction they look they'll think that they are in the center of a sphere of radius 13.5 billion light years long.

Yes, but only in their 'now', which is in our future light cone. We are 'now' at the edge of their observational universe and appear to be much younger than they.

So my conclusion to this is that when we talk of a different point in that sphere we're basically talking about traveling back in time since everything we see in the heavens is from the past. So to a point located at 1 million light years away from us that point exists 1 million years in the past. Correct?

So this bring me to my question if we were to travel back in time 13 billion years (in other words to a point 13 billion light years away from us) then the age of the universe would still be 13.5 billion years and not 0.5 billion years.

Where is the flaw (if any) in my logic?

Appreciate your comments.

How fast are you traveling? If you travel back in time [i.e., faster than the speed of light], the universe would appear to shrink. If you instantaneously traveled 13 billion light years, the universe would indeed appear to be only .7 billion light years in diameter. Turbo-1 gave a good explanation. I'm just using a slightly different approach.

 

[Entropy]

And as we discussed in another post, quasars (even at redshifts z~6.5) exist in hosts that exhibit metallicities in the solar and super-solar range. How can an object less than a billion years old have a metallicity equivalent to that of our own galaxy, which apparently has had 13.7Gy of stellar metal-enrichment to produce the chemical properties of our "neighborhood"? If quasars could condense out of materials in our galactic neighborhood, their high metallicities would be non-controversial, but it we believe that heavy elements (heavier than could be produced by BB Nucleosynthesis) are produced by fusion in stars, it is VERY difficult to explain the high metallicities of these early quasars. By this measure alone, the universe is apparently much older than 13.7Gy - MUCH older.

This is why I think the universe is much older than 13 billion years old. But exactly how old, I can't give even an educated guess.

 

[Chronos]

There is nothing yet suggesting metallicity in the early universe is a significant theoretical problem:

The cosmological evolution of metal enrichment in quasar host galaxies
http://arxiv.org/abs/astro-ph/0309533

In both the models and observations, the rate of evolution of the mean quasar metallicity as a function of redshift is generally flat out to z ~ 4-5. Beyond the observed redshift range and out to redshift z ~ 6-8, we predict a slow decline of the mean central metallicity towards solar and slightly subsolar values as we approach the epoch of the first significant star formation activity

The Era of Massive Population III Stars: Cosmological Implications and Self-Termination
http://arxiv.org/abs/astro-ph/0310443

Using the results from previous high-resolution cosmological simulations of early structure formation that include radiative transfer, we show that a significant volume fraction of the IGM can be metal-polluted, as well as ionized, by massive Population III stars formed in small-mass (10^6-10^7 Msun) halos early on. If most of the early generation stars die as pair-instability supernovae with energies up to 10^{53} ergs, the volume-averaged mean metallicity will quickly reach Z ~ 10^{-4}Zsun by a redshift of 15-20

 

[turbo]

In both the models and observations, the rate of evolution of the mean quasar metallicity as a function of redshift is generally flat out to z ~ 4-5. Beyond the observed redshift range and out to redshift z ~ 6-8, we predict a slow decline of the mean central metallicity towards solar and slightly subsolar values as we approach the epoch of the first significant star formation activity.

I italicized the important part of that quote. The authors of that paper predict a slow decline in the metallicity, because if that decline is not observed, the BB model is dead. So far (to z~6.5) no decline is observed. In fact, the scatter of quasar metallicities seems unrelated to redshift (paper cited above).

 

[Chronos]

Agreed, by their argument. Do you have evidence refuting that position?

 

[A_I_]

i've read some replies on that topic and some of you took our Earth as a point of reference to measure.
so the real age of the Universe is not what we calculate because obviously relativity plays its role in everything therefore if we come to a consensus it would be only for us; i mean the age will only be relative to our beliefs in our theories and our calculations.
Talking about the universe and its radius;
well as we r prooving it's expanding
we can not consider a constant radius;
and i believe (i don't know if anyone shares this idea with me)
that each point of the universe is its center with respect to that point.
Basing my judgement on a fact not totally prooven but might be a right answer which is that our Universe is expanding and unlimited.
NO ONE can imagine infinity
thus; no one can imgine the center of an infinite SPHERE (the universe)
this is in the case if our Universe is spherical.
Too many questions.
Hopefully we will reach the speed of light,
maybe by then we can answer a small part of our infinite questions!

 

[turbo]

Agreed, by their argument. Do you have evidence refuting that position?

Yes.

http://cosmos.as.arizona.edu/~thompson/pubdb/docs/barth03a.pdf

 

[Chronos]

Insufficient. That does not invalidate the model. From the paper you cited:

The Fe ii/Mg ii ratio is also similar to values found for lower redshift quasars, demonstrating that there is no strong evolution in Fe/a broad-line emission ratios even out to z ~ 6.4. In the context of current models for z p 6.4 iron enrichment from Type Ia supernovae (SNe Ia), this implies that the SN Ia progenitor stars formed at z ~ 10.

This same conclusion has been already been derived numerous times. Until the iron lines are pushed out past z = 8, this is old news.

 

[turbo]

Insufficient. That does not invalidate the model. From the paper you cited:

The Fe ii/Mg ii ratio is also similar to values found for lower redshift quasars, demonstrating that there is no strong evolution in Fe/a broad-line emission ratios even out to z ~ 6.4. In the context of current models for z p 6.4 iron enrichment from Type Ia supernovae (SNe Ia), this implies that the SN Ia progenitor stars formed at z ~ 10.

This same conclusion has been already been derived numerous times. Until the iron lines are pushed out past z = 8, this is old news.

These folks feel that the metallicities (especially iron abundance) of quasars at z~6 are higher than can be explained by the standard model.

http://citebase.eprints.org/cgi-bin/fulltext?format=application/pdf&identifier=oai%3AarXiv.org%3Aastro-ph%2F0406630

 

[RoboSapien]

I am an amature but here is a question that is bloging me.

Universe may be 15 billions lightyears wide but to get that wide, did it happen at light speed ? If not then how many years should it take to be so ?

 

[Chessguy]

Another amateur question

let us suppose that we are exactly at the centre of a spherical universe
if we look in one direction and see stars, microwaves or whatever for 6.85 billion light years then nothing
then we turn around and look in the opposite direction and see stuff for 6.85 billion light years then nothing-ness?
If we could see that much stuff then wouldn't that mean that because this light that we can see really eminated from its space 6.85 billion years ago so if it is expanding at the speed of light (I don't know this just asking?) then it has really expanded another 6.85 billion light years in the time it took its light to travel to us? that would make that universe 13.7 + 6.85 = 20.55 billion light years old?

I'm guessing that the people calculating this stuff would have factored that in - so we really only must be able to see stars/microwaves that are 4.57 billion light years away in each direction for the universe to be 13.7 billion years old? assuming it is expanding at the speed of light and we are the centre of the universe of course?

 

[RoboSapien]

Chessguy

Wow, U mean that what we see 15 billion years old has already moved on ahead of that distance since its light reached us.

That means I should ask How long did it take for the universe to be 15+ Billions light years wide depending on the speed of its growth.

I am not an expert but here is my suggestion for the answer of my above question and please correct me because I am sure I am wrong. If we measure the speed of farthest objects in opposite directions of the visible universe then we should get a rough idea of the speed of expansion of universe then we can calculate how long it would have taken to be 15 billion light years wide.

But wait, I also wana know if there any two object that are 15+ billion light years away and in opposite direct of the visible universe because that will make it 30billions light years wide. I am a fool and sorry about being wrong.

 

[Chronos]

These folks feel that the metallicities (especially iron abundance) of quasars at z~6 are higher than can be explained by the standard model.

http://citebase.eprints.org/cgi-bin/fulltext?format=application/pdf&identifier=oai%3AarXiv.org%3Aastro-ph%2F0406630

Again, this is an unanswered question, not a stake through the heart. From that paper:

We investigate the relationship between Fe/Mg and Fe/C abundance ratios... which have been measured in QSOs out to redshifts of approximately six... the values of these ratios measured in objects at redshifts of approximately six suggest that iron enrichment has occurred more rapidly in these objects than predicted by the assumed elliptical starburst model, under currently favored cosmologies.

Luminosity and metallicity evolution is addressed in numerous studies- e.g.:
http://www.stsci.edu/stsci/meetings/shst2/griffithsr.html
http://arxiv.org/abs/astro-ph/0307014
The success or failure of the elliptical starbust, or other such models, does not fundamentally challenge the big bang model - only evolutionary models. Of this we have an admittedly poor understanding, so it is not very surprising to find surprises. BBN, however, is not yet in peril.

 

[turbo]

RoboSapien and Chessguy, maybe this Ned Wright lecture will help with the concepts.

http://online.itp.ucsb.edu/online/colloq/wright1/

Click on the webcam link corresponding to your available bandwidth.

 

[Chronos]

I second turbo-1's endorsement. Ned Wright is a very good source. Here is another link if that one is too slow for your taste. Try the tutorial:
http://www.astro.ucla.edu/~wright/cosmolog.htm

 

[guevaramartyr]

In our universe, we have observed that cause and effect is always true. Therefore, the Universe could not have simply sprung into being 13.5 billion or however many years ago. Thus, the only logical conclusion is that the universe is a kind of vacuum diagram, e.g. it initiates its own beginning. In my belief, infinitely far in the future, the future will become the past, thus proposing time as a closed loop. I imagine that far in the future, when the black holes have consumed all matter and combined, when the energy from the Big Bang is expended, only a singer super-black hole will exist. This is the point when the proposed white hole will occur, a kind of reversing of the process where matter is released from the gravity well. IMO, the Big Bang was one of these white holes, and so the universe slowly "breathes", expanding and contracting over an infinite amount of time. It has always been here and always will be. Given this infinite amount of time, all logical things will come to pass.

Comments?

 

[Chronos]

In our universe, we have observed that cause and effect is always true. Therefore, the Universe could not have simply sprung into being 13.5 billion or however many years ago. Thus, the only logical conclusion is that the universe is a kind of vacuum diagram, e.g. it initiates its own beginning. In my belief, infinitely far in the future, the future will become the past, thus proposing time as a closed loop. I imagine that far in the future, when the black holes have consumed all matter and combined, when the energy from the Big Bang is expended, only a singer super-black hole will exist. This is the point when the proposed white hole will occur, a kind of reversing of the process where matter is released from the gravity well. IMO, the Big Bang was one of these white holes, and so the universe slowly "breathes", expanding and contracting over an infinite amount of time. It has always been here and always will be. Given this infinite amount of time, all logical things will come to pass.

Comments?

That is within the realm of possibility. It is called the cyclical universe. There are three basic versions at present - the closed universe 'big crunch' model; the open universe 'big rip' model; and the more recent flat universe 'quantum fluctuation' model.

 

[Entropy]

In our universe, we have observed that cause and effect is always true.

Not in QM.

It has always been here and always will be. Given this infinite amount of time, all logical things will come to pass.

That's not really logical. If time goes infinitely backwards and forwards, then how did we get to this point in time? Time needs a begining, but not necessarily an end.

 

[turbo]

Not in QM.

Absolutely! At some level, classical determinism gives way to amplitudes of probability. Cause and effect is turned on its head when you look at Feynman diagrams and see a particle traveling backward in time.

That's not really logical. If time goes infinitely backwards and forwards, then how did we get to this point in time? Time needs a begining, but not necessarily an end.

Temporal infinity (in either direction) is no more logical or illogical than spatial infinity. It just seems to be a human NEED to define a beginning. For some reason, people can easily imagine time rolling along forever in the future, but many have trouble with imagining that time has always been rolling along and never had a beginning.

 

[guevaramartyr]

I apologize for my earlier comment. While quantum mechanics can violate the laws of cause and effect, I guess it just seems to make more sense to me that the universe as a whole obeys the laws of cause and effect. After all, at the macro level, most things in the universe seem to follow cause and effect, at least as far as our limited knowledge tells us. Quantum physics violates so many of the macro laws that is almost seems not to fit. Until more detailed knowledge of the quantum universe can be obtained I guess we're all just entitled to our opinions about the nature of reality, arent we?

 

[turbo]

I apologize for my earlier comment. While quantum mechanics can violate the laws of cause and effect, I guess it just seems to make more sense to me that the universe as a whole obeys the laws of cause and effect.

Do not apologize. We live in a macroscopic world of cause and effect, and as others (Feynman among them) has said, if you think you understand quantum mechanics, you are mistaken.

Until a more detailed knowledge of the quantum universe can be obtained I guess we're all just entitled to our opinions about the nature of reality arent we?

We are all entitled to our own opinions about the interpretation of the observations, but we are not each entitled to our own sets of observations, unless they can be duplicated and verified scientifically.

Ultimately, quantum theory and general relativity will be reconciled. My personal view is that we will have to fit GR into Euclidean space and provide a real mechanical process that explains gravitational attraction, instead of the ad-hoc mathematical approximation that "mass curves spacetime".

That mathematical curved space-time approximation works (with extreme accuracy) on very simple models, but it does not work on larger scales (galactic or cluster scales) without LOTS of undetectable non-baryonic dark matter. In the standard model, dark matter is not only a LOT more prevalent than the matter that we can see through its emisssions, reradiated energy, etc, but it also must distribute itself "just so" in order to effect the observed galactic rotations, gravitational binding of clusters, etc, to keep the Big Bang viable.

Looking beyond the academic credentials of the people proposing this stuff, we are apparently now back at the point where we need to propose "angels" and argue about how they might arrange themselves and how hard they can push or pull on the celestial bodies to make our observed universe comply with our "accepted model" of the universe. This silliness will pass - the question is when will it pass? I would hate to die in the dark ages!

Dark matter is not and has never been detectable. Dark energy is not and has never been detectable. There is no way to experimentally prove or falsify early inflation. All these improbable/impossible things are needed to keep the Big Bang model alive, and none of them have been experimentally sensed, despite years of dedicated effort by MANY eager young researchers who would love to get the King of Sweden's meatball recipe! It is time to consider that the BB model is wrong.

 

[Chronos]

Do not apologize. We live in a macroscopic world of cause and effect, and as others (Feynman among them) has said, if you think you understand quantum mechanics, you are mistaken.

We are all entitled to our own opinions about the interpretation of the observations, but we are not each entitled to our own sets of observations, unless they can be duplicated and verified scientifically.

Ultimately, quantum theory and general relativity will be reconciled. My personal view is that we will have to fit GR into Euclidean space and provide a real mechanical process that explains gravitational attraction, instead of the ad-hoc mathematical approximation that "mass curves spacetime".

That mathematical curved space-time approximation works (with extreme accuracy) on very simple models, but it does not work on larger scales (galactic or cluster scales) without LOTS of undetectable non-baryonic dark matter. In the standard model, dark matter is not only a LOT more prevalent than the matter that we can see through its emisssions, reradiated energy, etc, but it also must distribute itself "just so" in order to effect the observed galactic rotations, gravitational binding of clusters, etc, to keep the Big Bang viable.

Looking beyond the academic credentials of the people proposing this stuff, we are apparently now back at the point where we need to propose "angels" and argue about how they might arrange themselves and how hard they can push or pull on the celestial bodies to make our observed universe comply with our "accepted model" of the universe. This silliness will pass - the question is when will it pass? I would hate to die in the dark ages!

Dark matter is not and has never been detectable. Dark energy is not and has never been detectable. There is no way to experimentally prove or falsify early inflation. All these improbable/impossible things are needed to keep the Big Bang model alive, and none of them have been experimentally sensed, despite years of dedicated effort by MANY eager young researchers who would love to get the King of Sweden's meatball recipe! It is time to consider that the BB model is wrong.

You had to know I couldn't let that pass without comment . As defense counsel for my client, the big bang model, I move for dismissal. My client is not subject to the statutes cited in charges filed by my esteemed colleague. Dark matter and dark energy are required to comply with GR, not BB, and inflation is neither required or forbidden.

The big bang model makes four fundamental predictions:
1] the universe has expanded over time
2] the universe is isotropic [looks the same everywhere on large scales]
3] the cosmic microwave background [CMBR]
4] primordial elemental abundance [BBN - big bang nucleosynthesis]
Observational support is both extensive and compelling.

Here is the short list of what is required to prosecute BB:
1] explain cosmological redshift without expansion
2] explain large scale gaussian distribution of matter and radiation
3] explain the CMBR and it's temperature without a hot big bang
4] explain light element abundance without big bang nucleosynthesis

Nearly all existing mysteries arise when you apply the rules of other theoretical models - general relativity, EM, quantum theory, particle physics, etc. - to observation. However few, if any, pose any threat to the standard big bang model.

 

[Garth]

Has the universe expanded over time? It depends on what ruler you use to measure it.
An 'atomic' ruler using the size of an atom as the standard of length would measure an expanding universe.
A 'photonic' ruler using the wavelength of a 'cosmological' photon (e.g. sampled from the CMB) as the standard of length would measure the universe to be static. Photons 'expand' with the universe even in GR.

But how do you explain cosmological redshift without expansion? The measurement of gravitational and cosmological redshift is a comparison of the energy of the photon with the mass of the atom it interacts with. If the wavelength of a photon is defined to be the standard of length then its energy is conserved. (Why should photons on null-geodesics crossing curved space-time with no other interactions lose energy?) If so then the mass of the atom it interacts with must have increased. Now we are no longer in GR but some other gravitational theory, such as the Jordan conformal frame of http://www.kluweronline.com/oasis.htm/5092775.

Garth

 

[turbo]

Dark matter and dark energy are required to comply with GR, not BB, and inflation is neither required or forbidden.

Gotta go to work, but quickly, inflation is absolutely required in the standard model to provide the isotropy in your [2].

The BB theory did not predict inflation. Inflation was required to rescussitate the BB model, and is a patch (epicycle), not a predictive success.

The BB model did not predict that the universe has expanded over time. That's putting the cart before the horse. The BB/expanding universe was proposed after Hubble and others observed that generally more distant objects have higher redshifts. Hubble was not very comfortable with the redshift=expansion concept, but the BB was a whiz-bang idea that captured popular sentiment, and it is presently being promoted as "true", "proven", etc. BB theory began as an extrapolation from the redshift=expansion idea, not the other way around.

 

[selfAdjoint]

The BB theory did not predict inflation. Inflation was required to rescussitate the BB model, and is a patch (epicycle), not a predictive success.

There is a subtle but important difference between something like inflation and an epicycle. An epicycle closes out discussion, it does not exist in "explanation space" but in "calculation space". Epicycles, and deferents, and the whole ptolemaic megillah, were never proposed as explanations or required to sustain any. Ptolemy specifically said the "causes"were the province of phlosophers and he was just proposing a calculation system to "save the appearances", i.e compute the positions of the heavenly bodies.

On the other hand inflation came originally with a physical explanation; that turned out to have gaps and so there has been a lot of physical research to find the correct explanation. Nobody (except maybe you and your friends) regards it as just a toss-away. And now it has become an important test for theories of quantum gravity. Those which can account for inflation have crowing rights over thse which cannot.

In general, a coarse view could regard almost any scientific innovation as "an epicycle", since they are typically a response to some failing of existing theory. If there weren't any failing, there wouldn't be room for any new physics!

 

[Garth]

On the other hand inflation came originally with a physical explanation.....In general, a coarse view could regard almost any scientific innovation as "an epicycle", since they are typically a response to some failing of existing theory. If there weren't any failing, there wouldn't be room for any new physics!

So where is the Higgs boson?

Garth

 

[turbo]

On the other hand inflation came originally with a physical explanation; that turned out to have gaps and so there has been a lot of physical research to find the correct explanation. Nobody (except maybe you and your friends) regards it as just a toss-away. And now it has become an important test for theories of quantum gravity. Those which can account for inflation have crowing rights over thse which cannot.

In general, a coarse view could regard almost any scientific innovation as "an epicycle", since they are typically a response to some failing of existing theory. If there weren't any failing, there wouldn't be room for any new physics!

I don't regard inflation as a toss-away, but as a "patch" required to keep the BB universe homogeneous and isotropic. If someone working on quantum gravity can explain a mechanism by which inflation starts (and stops, by the way) at the appropriate times to explain these observed qualities without having to invent additional entities and events, he or she will have accomplished something wonderful.

One reason why I find the BB model so suspect is that it requires many mysterious entities and events in order to stay viable. Paraphrasing Richard Feynman: Nature is complex, but the underlying rules are simple. In the standard model, complexity, unexplained behavior, and unobserved entities abound. This is not an encouraging sign.

 

[Chronos]

I don't regard inflation as a toss-away, but as a "patch" required to keep the BB universe homogeneous and isotropic. If someone working on quantum gravity can explain a mechanism by which inflation starts (and stops, by the way) at the appropriate times to explain these observed qualities without having to invent additional entities and events, he or she will have accomplished something wonderful.

One reason why I find the BB model so suspect is that it requires many mysterious entities and events in order to stay viable. Paraphrasing Richard Feynman: Nature is complex, but the underlying rules are simple. In the standard model, complexity, unexplained behavior, and unobserved entities abound. This is not an encouraging sign.

It is fair to characterize inflation as a patch, but I empathize with SA's discomfort in terming it an epicycle. It suggests adding complexity to rescue a flawed theory. Inflation simplifies the model by explaining flatness, homogeneity, high energy relic particles, and smoothly leads to big bang nucleosynthesis. Inflation has plenty of theoretical support. It occurs in many models, depending on initial conditions - and therein lies the rub. It emerges in models that are mutually exclusive and the permissible range of initial conditions is too broad to distinguish which models are viable and which are not. It is logical to continue gathering clues to narrow those ranges.

I do not understand your assertion the big bang model "requires many mysterious entities and events in order to stay viable". Please be specific. As previously noted, you cannot undermine the big bang model using dark matter, dark energy or Higgs bosons without turning GR and particle physics models on their heads as well. That's a lot to put on the same plate.

 

[turbo]

I do not understand your assertion the big bang model "requires many mysterious entities and events in order to stay viable". Please be specific. As previously noted, you cannot undermine the big bang model using dark matter, dark energy or Higgs bosons without turning GR and particle physics models on their heads as well. That's a lot to put on the same plate.

Perhaps I should have said "standard model" instead, to be very inclusive, but the BB is inextricably intertwined with GR. It is a model of an emergent universe in GR space-time. You could just as well posit an emergent universe in Euclidean space, or perhaps a steady state universe in either of those two geometries.

As for the requirements of the BB model, the putative present-day accelerated expansion of the BB universe requires dark energy of some kind regardless of the geometry (GR, Euclidean, Klingon ). GR does not demand this acceleration, nor does particle physics, QED or any other field of science - just BB cosmology.

 

[moving finger]

I don't regard inflation as a toss-away, but as a "patch" required to keep the BB universe homogeneous and isotropic. If someone working on quantum gravity can explain a mechanism by which inflation starts (and stops, by the way) at the appropriate times to explain these observed qualities without having to invent additional entities and events, he or she will have accomplished something wonderful

I guess you regard inflation as a "patch" because inflation does not follow as a necessary consequence of other more accepted fundamental physical theories? This is as it may be, but the fact remains that inflation does neatly explain a number of features of our universe which cannot be easily explained in other ways. Not only the observed homogeneity and isotropy of the universe, but also why the actual mass density appears so close to the critical mass density, and why the universe apparently started in a very unusual entropy state (the "unreasonable aim of the creator" which Penrose cannot explain in any other way - though Penrose himself seems to be an inflationary sceptic).

Is there perhaps an alternative to inflation which explains homogeneity, isotropy, critical mass and initial entropy as neatly and simply as inflation does?

 

[Garth]

Is there perhaps an alternative to inflation which explains homogeneity, isotropy, critical mass and initial entropy as neatly and simply as inflation does?

Yes freely coasting cosmolgy , which requires a mechanism to deliver the strictly linear cosmological expansion (Kolb's K-matter in A coasting cosmology ) that is provided by Self Creation Cosmology.

Inflation explains critical mass only if combined with Dark Energy. The WMAP data is consistent not only with flat geometry but more generally with conformally flat geometry, it being angular in nature, but a conformally flat model, such as the freely coasting model, would not need DE. It also identifies DM as well as being baryonic. Occam's razor anyone?

 

[moving finger]

Inflation explains critical mass only if combined with Dark Energy. The WMAP data is consistent not only with flat geometry but more generally with conformally flat geometry, it being angular in nature, but a conformally flat model, such as the freely coasting model, would not need DE. It also identifies DM as well as being baryonic. Occam's razor anyone?

Thanks for the leads. I'll check them out.
But I'm not sure that Dark Energy has anything to do with critical mass? I think you mean Dark Matter? In fact inflation predicts that the mass density of the universe should be close to the critical mass density and therefore it "requires" Dark Matter (or something similar) to exist.

 

[Garth]

But I'm not sure that Dark Energy has anything to do with critical mass? I think you mean Dark Matter? In fact inflation predicts that the mass density of the universe should be close to the critical mass density and therefore it "requires" Dark Matter (or something similar) to exist.

Inflation predicts exact critical density.

However observations of gravitational lensing and cluster dynamics can provide only 1/3 and 1/5 critical density respectively. But the WMAP data is consistent with a flat geometry i.e. exact critical density. Furthermore the recent distant S/N Ia observations indicated cosmological acceleration.

So the shortfall of ~2/3 critical density was made up by a new concept, DE, which might also then be conscripted to explain the acceleration as well.

However nobody knows what DE is, although there are plenty of ideas, (leaky gravity anyone?) and therefore it might be considered a 'patch' or 'epicycle' introduced to save the standard model.

Observed density is only about 0.4% critical density max, and the maximum baryonic density coming out of the standard BBN is only 4% critical density so DM is required to make up the difference to 27% as required by modelling of the WMAP data.

That there are published alternatives that do not require such patches (although they may have their own individual problems) ought to be more readily acknowledged.

Garth

 

[SpaceTiger]

Inflation explains critical mass only if combined with Dark Energy.

Though it's also worth noting that dark energy potentially explains the supernovae measurements as well. I share your skepticism of the standard model, but to be fair, I think it contains the fewest epicycles/patches/tooth fairies of all currently available theories.

 

[moving finger]

Inflation predicts exact critical density.

not wishing to seem pedantic, the critical density is a real number. inflation forces the mass/energy density to be asymptotically close to this critical density by multiple (but limited) number of doublings of space. The actually mass/energy density would only become exactly equal (in a strictly mathematical sense where real numbers are concerned) to the critical density if those doublings continued indefinitely, which they did not. We must therefore accept the possibility (even with inflation) that the actual mass/energy density is not "exactly" equal to the critical mass density

However observations of gravitational lensing and cluster dynamics can provide only 1/3 and 1/5 critical density respectively. But the WMAP data is consistent with a flat geometry i.e. exact critical density.

again i take issue with your use of the word "exact" in this context. Do you know the error bars involved? (the error bars need to be literally of zero length for an exact match)

However nobody knows what DE is, although there are plenty of ideas, (leaky gravity anyone?) and therefore it might be considered a 'patch' or 'epicycle' introduced to save the standard model.

Your original post regarding "patches" on this thread referred to Inflation as a patch, which is where I joined the discussion. Any concept which is not predicted or required by accepted basic theory can be considered a patch - so both Dark Matter and Dark Energy are also "patches" in this sense.

Dark Energy can be considered to be the vacuum energy (the energy contained within the vacuum) with an equation of state given by pressure = - energy density (for matter the equation of state tells us that pressure is independent of energy density, and for radiation pressure = 1/3 x energy density). Negative pressure leads to repulsion (this was the source of the original cosmological constant in Einstein's equations), hence vacuum energy causes the universe to expand.

 

[Chronos]

One thing I find striking about inflation: It suggests the universe has and will always be within a quantum fluctuation of perfectly flat. That is compelling. It is too much a coincidence that the universe appears to be almost perfectly flat now, unless it was always flat.

 

[SpaceTiger]

One thing I find striking about inflation: It suggests the universe has and will always be within a quantum fluctuation of perfectly flat.

Well, that depends on the cosmological parameters of the universe. In a simple open (omega_tot<1) matter-dominated model, the comoving Hubble radius increases and eventually encompasses the whole of the pre-inflation universe. The geometry pre-inflation is completely unknown, so in this case, the universe couldn't be guaranteed to always be flat. If, however, we enter another era of accelerated expansion (as would be the case with a cosmological constant), then the universe would actually get more flat with time because the comoving Hubble radius would decrease.

 

[moving finger]

One thing I find striking about inflation: It suggests the universe has and will always be within a quantum fluctuation of perfectly flat. That is compelling. It is too much a coincidence that the universe appears to be almost perfectly flat now, unless it was always flat.

I don't think so. The energy/mass density may have been driven extremely close to the critical density (ie Omega was almost exactly equal to 1) during inflation (ie the universe was almost perfectly flat at the end of inflation, with a value of Omega equal to 1 within 15 decimal places according to Alan Guth), but what evidence do we have that it remains perfectly flat today, or will continue to be flat in the future? The experimental evidence is that the CMB is isotropic only to one part in 10^5, and the latest WMAP observations have an uncertainty in Omega of plus or minus 0.02 (see http://www-ctp.mit.edu/~guth/iccs/iccs-guth.pdf ). All we can conclude from this is that the universe is "almost" flat now, but certainly not "within a quantum fluctuation of perfectly flat".

It is too much a coincidence that the universe appears to be almost perfectly flat now, unless it was always flat.

No need to invoke coincidences. The explanation is inflation, which drove the universe to almost perfect flatness at the time of the Big Bang; but that does not rule out subsequent deviations from flatness.

 

[SpaceTiger]

No need to invoke coincidences. The explanation is inflation...

Well, let's be a little careful there. Inflation is an extremely nice solution to a lot of problems, but it has yet to make any really convincing predictions. There may be other nice solutions to the flatness and horizon problems that we simply haven't thought of yet. The trouble with inflationary theory is that it can produce virtually any final result if you tweak the parameters enough.

Besides, we're still struggling to explain how inflation stopped.

 

[moving finger]

Well, let's be a little careful there. Inflation is an extremely nice solution to a lot of problems, but it has yet to make any really convincing predictions.

Incorrect. Inflation theory was first developed around 1980, when many cosmologists did not accept the universe is necessarily flat and isotropic, and there certainly was little experimental evidence to suggest that it was either flat or isotropic. Inflation was developed precisely because the conventional Big Bang model was inadequate to properly explain the universe - that's exactly why new theories are developed in the first place. Inflation predicts that the universe will be flat and isotropic, and the really good experimental evidence confirming this is the case has come in only in the last couple of years (WMAP data), long after the theory was established.

There may be other nice solutions to the flatness and horizon problems that we simply haven't thought of yet. The trouble with inflationary theory is that it can produce virtually any final result if you tweak the parameters enough.

There may be, and there may not be, other solutions. Until someone comes up with a better explanation, which fits the facts as well or better than inflation, then we can either say "I have no idea" or "inflation provides a good explanation". I know which answer I prefer.

Besides, we're still struggling to explain how inflation stopped.

And your point is? Just because a theory does not provide all of the answers is not a reason to reject it as a good explanation for the answers it does provide.

I can imagine there were sceptics around in Newton's time who said " let's be a little careful, there may be other solutions to the gravity problem we haven't thought of yet"... of course Newton was not entirely correct, and he could not explain where gravity "came from", but his ideas and insight advanced science and mechanics for the next 200 years.

 

[SpaceTiger]

Incorrect. Inflation theory was first developed around 1980, when many cosmologists did not accept the universe is necessarily flat and isotropic

You're joking, right?

His original paper was entitled: Inflationary universe: A possible solution to the horizon and flatness problems

By the way, isotropy is not a prediction of inflation, it's a generic property of universes that obey the cosmological principle.

Inflation was developed precisely because the conventional Big Bang model was inadequate to properly explain the universe - that's exactly why new theories are developed in the first place. Inflation predicts that the universe will be flat and isotropic, and the really good experimental evidence confirming this is the case has come in only in the last couple of years (WMAP data), long after the theory was established.

I think you need to brush up on your cosmology history a bit. WMAP gave us very precise measures of the cosmological parameters, but their orders of magnitude (all that's needed to determine that the problems existed) have been known for decades.

There may be, and there may not be, other solutions. Until someone comes up with a better explanation, which fits the facts as well or better than inflation, then we can either say "I have no idea" or "inflation provides a good explanation". I know which answer I prefer.

I didn't say there was anything wrong with inflation, I just said that it's not yet on an entirely solid foundation. You shouldn't simply assume theories are right simply because you haven't thought of an alternative. There's a long history of such mistakes in physics.

Besides, I'm not saying "I have no idea", I'm saying that inflation is not a sure thing, as you seemed to suggest.

And your point is? Just because a theory does not provide all of the answers is not a reason to reject it as a good explanation for the answers it does provide.

I'm not rejecting it, I'm saying that we need to converge on a stopping mechanism before we can say that it's true beyond reasonable doubt. Besides, the lack of a stopping mechanism is not a failure to explain something, it's an inherent problem with the model.

I can imagine there were sceptics around in Newton's time who said " let's be a little careful, there may be other solutions to the gravity problem we haven't thought of yet"...

I certainly hope that there were. Such skepticism is extremely helpful for the scientific process.

 

[moving finger]

WMAP gave us very precise measures of the cosmological parameters, but their orders of magnitude (all that's needed to determine that the problems existed) have been known for decades.

At the time Guth first published there were certainly those who "believed" the universe was flat and isotropic, but there was very little real evidence that this was indeed the case. Inflation predicted that the universe should be very flat, not just within "orders of magnitude" - and the observational verification of this has only been made very recently. Inflation was developed as the explanation of a perceived problem, and since its development the magnitude of the "problem" has been confirmed to be in perfect agreement with the predictions of inflation.

By the way, isotropy is not a prediction of inflation, it's a generic property of universes that obey the cosmological principle.

The Cosmological Principle is a principle, not a theory, and since it is a principle it makes no predictions. Any cosmological theory must obey the Cosmological Principle otherwise it is doomed to failure. The predictions of inflation obey the Cosmological Principle. Isotropy is one prediction of the theory of inflation. Isotropy may also be a prediction of other theories, but that doesn't invalidate the claim that it is predicted by inflation.

I didn't say there was anything wrong with inflation, I just said that it's not yet on an entirely solid foundation. You shouldn't simply assume theories are right simply because you haven't thought of an alternative. There's a long history of such mistakes in physics.

Science proceeds by people advancing hypotheses to explain the facts, and those hypotheses then being (possibly) disproven. No hypothesis can ever be proven to be 100% correct, the best any hypothesis can hope for is to stand the test of time by agreeing with observations and not being proved incorrect. This is where inflation is right now. Its standing up there waiting to be shot down, and so far nobody has offered up any viable contenders or shot it down.

I'm saying that inflation is not a sure thing, as you seemed to suggest.

You're misreading me. I never said inflation is a "sure thing" - what I am saying is its the best candidate we have right now. If you know of a better candidate then I'm sure we would all love to hear about it.