Is the universe 13.7 Billion years old? There seems to be a contradiction!

surajt88

The universe is said to be 13.7 Billion years (check). The farthest object that has ever been seen is UDFj-39546284 at a distance of 13.2 Bn light years. It is also known that all observable objects were constituted within a very small region during the Big Bang.
So, for the present galaxy UDFj-39546284 and the present Earth to have traveled as far as they are now, and even if we assume that the big bang occurred right between Earth and UDFj-39546284, it must have taken at least 6.6 Billion years for the objects to have been positioned as they are now (Even if they are assumed to have traveled at the speed of light). The light that left the galaxy UDFj-39546284 6.6 Billion years after the Big Bang (as we see now) should have taken another 13.2 Billion years to reach us.
So the universe must be at least 19.8 Billion years old now. Where am I wrong?

Drakkith

You havn't taken inflation and expansion of the universe into account.

Dmitry67

What do you mean by "big bang occurred right between Earth and UDFj-39546284"?

No point of space is special, there is no spacial location where Big bang had occured. It had occured everywhere

surajt88

Thanks a lot...But both earth and that galaxy are receding each other now. I would suppose both(considering earth and the galaxy are two points in space) must have been very close at some point in time. How long would you suppose it took for the two points to be in the current position from being very close. The light rays that left the galaxy then took 13.2 Billion light years to reach earth.
Of course I haven't taken into account inflation and the expansion of the universe. May I know how it affects what I mentioned above.

Drakkith

When that light left that galaxy it would not have been 13.6 billion light years away, but probably MUCH closer. Something like 500 million light years perhaps. Thanks to inflation and expansion, that light has taken 13.2 billion years to reach us since we were speeding away and it had to catch up.

Edit: I think this is correct. I'm really tired due to being on shift for 12 hours now, so I can't really think that clearly at the moment. About to get off though!

Bill_K

This discussion has already gotten out of hand, so let me start over. In the first place the currently accepted age of the universe is 13.75 Gy, not 13.7.

Secondly, yes when the light from that galaxy was emitted 13.6 Gy ago, it *was* 13.6 Gy away. Light, after all, travels at the speed of light!

However, the galaxy did not take 13.6 Gy to arrive at its position. Immediately following the Big Bang was a period of cosmic inflation, in which space itself underwent a rapid exponential expansion. The expansion of space is not limited to light speed.

Nabeshin

Inflation is completely unnecessary and irrelevant to this discussion. Since inflation occurred when the universe was but a tiny quark-soup of sorts, there were certainly no galaxies around then to be emitting light. So we can safely start our analysis in a post-inflationary era where the universe is described by a normal ΛCDM model.

Ok, let's take UDFj-39546284 as an example where redshift z~10. This means that when light from this galaxy which is just now reaching us was emitted, the universe was ~11 times smaller than it is today. So what you should imagine is the two objects, namely ourselves and UDFj-39546284 much closer at the beginning of the universe, ~300Myr after the big bang. UDFj-39546284 emits some light, which begins traveling through space towards us. Since the universe is expanding (and also flat), it takes much longer than you would naively think for the light to actually reach us. You can think of, if you like, extra "space" being created between the two objects which the beam of light must now traverse. The net effect here is that when the light finally reaches us, it has traveled perhaps the 1 billion years (made this number up. Someone can calculate the actual comoving distance if they want) it might have, but over 13 billion years.

Also note that in cosmology there is some ambiguity as to what is meant by "distance". For example, there are luminosity distances, comoving distances, and angular diameter distances, none of which are the same when we are looking at distant objects.

Drakkith

So light from a galaxy that is 13 billion LY away was emitted...when?

surajt88

I seem to be getting closer to understanding this now. I'll put it in my words. Plz correct me if I'm wrong.
1. Big bang occurs.
2. After 'X' billion years, UDFj-39546284 and the point in space which is to become earth now, are 'A' light years apart. The light which we see now, left the galaxy then.
3. Universe continues to expand.
4. 13.75 billion years after the big bang, UDFj-39546284 and the earth , are 13.2 billion light years apart. The light reaches us now.
5. So, if we know the present rate of increase in distance between the two, we may calculate the distance between them when the light left the galaxy.

So far so good...
As far as i know, the distance is calculated based on the red shift when the light left the galaxy (i.e when they were 'A' light years apart or around 480 million years after the big bang). So 'A' must be 13.2 billion light years as we see now.
Also the time the light took to reach us cannot be determined without considering the rate of expansion of the universe.

If the universe was 13.2billion light years in radius 480 million years after the big bang, I would suppose the universe must be older and bigger than what is said now.

qraal

Hi Guys

Best place to go to help illuminate this question is Prof. Ned Wright's Cosmology Tutorial...

Cosmology Tutorial

...where you'll learn the difference between the Light Travel Time and the Co-moving radial distance. Two quite different distances, often confused in the media. Using the Cosmology Calculator, with the input of a Light Travel-Time of 13.2 Gyr, one gets a co-moving radial distance of 31.69 Glyr. Why so much further away? Because the space-time between the two points has expanded while light has been traveling. Read more to learn just how that works in relativity.

Andrew Mason

I have yet to find a satisfactory explanation to the question you are asking.

Suppose a galaxy in a distant part of the universe travelling at .9998 x the speed of light ([itex]\gamma = 50[/itex]) away from earth (let's assume the earth began at the time of the big bang) sent a light signal toward the earth at a time, according to its own clocks, of BB+100m (100 million years after the Big Bang). Let's assume at the time of the Big Bang the galaxy and the earth were in the same place in space and 100 million years later, according to the galaxy clock, they are separated by d1 = .9998c x 100m = 99.98 million light years as measured by an observer in the galaxy.

According to the galaxy clock, that light should arrive at earth after another 99.98 million years have elapsed. By that time, the galaxy would be d2 = 100+99.98 x.9998 = 199.96 light years away from the earth as measured in the galaxy's frame of reference.

Now that same event (sending the light signal) would be observed by an earth observer as follows:

Time when light sent from galaxy: [itex]t = \gamma t_1'[/itex] = 50 x 100m = 5 billion years (after BB).

Separation of galaxy at that time: [itex]d = \gamma d_1'[/itex] = 50 x 99.98m = 4.999b (billion) light years.

Time on earth when light arrives: [itex]t = \gamma t_2'[/itex] = 50 x (199.98m) = 9.999b years.

So light from a galaxy that appears to us to be a tad less than 5 billion light years away when sent, arrives on earth a tad less than 10 billion years after the Big Bang as measured on the earth. If light arrives from a galaxy that was 13 billion light years away when it was sent, as measured by the earth, this means that it arrives a tad less than 26 billion years after the Big Bang.

So my question is: how can the age of the universe (in earth years) be much less than double the distance of the farthest galaxy/c ?

AM

Arbitrageur

correct, inflation has nothing to do with it.

Thanks for the good answer.

I agree, the 13.2Gyr is sometimes referred to as "lookback time", and is not the current distance which would be ~32Glyr.

qraal is correct, the distance now would be much greater, more like ~32Glyr.

If you had Earth A and Earth B 100 million light years apart both traveling in parallel paths away from the galaxy at 0.9998c and sending light beams to each other, then your math would be correct that the galaxy would observe that it would take 4.99 billion years for the light to travel that 100 million light year distance between Earth A and Earth B. But that's not the situation we have with the Earth receding from the galaxy. The effect of relativistic time dilation is to affect the frequency (or wavelength) of the light observed, not the time it takes to reach the observer since it wouldn't take more than 200 million light years from either frame of reference for the light to arrive if it left when the separation was 100 million light years, if space wasn't expanding.

But cosmologists believe that space itself is expanding, which isn't accounted for in your equations at all. The redshift we observe is largely the result of the expansion of space governed by general relativity, and also partly the result of the Doppler effect of special relativity. For a proper treatment of the effects of general relativity regarding a galaxy moving away from the Earth, see this paper:
A comparison between the Doppler and cosmological redshifts, by Maria Luiza Bedran
http://www.df.uba.ar/users/sgil/phys...r_redshift.pdf

That paper may also shed some light on the original question as suggested by the abstract:
"We compare the Doppler effect of special relativity with the cosmological redshift of general
relativity in order to clarify the difference between them. Some basic concepts of observational cosmology, such as the definitions of distance and cosmological parameters, are also presented."

Andrew Mason

I don't follow you there. Length contraction only applies in the direction of relative motion. There would be no length contraction perpendicular to the direction of travel.
I'm having trouble following you there too. If that was the case, then the observers on the earth would not measure the speed of light as c. Would that not violate the principle of relativity?

AM

bcrowell

A mistake that has been made at least once in this discussion is addressed on p. 10 of this article: http://www.mso.anu.edu.au/~charley/p...DavisSciAm.pdf See the box titled "How large is the observable universe?"

The use of Lorentz gamma factors in #11 doesn't actually work, since Lorentz transformations are local things. GR doesn't have the equivalent of a global Lorentz transformation.

Andrew Mason

This Scientific American article is a little short on detailed explanations of any of this. The following quote is particularly puzzling because it invokes a form of cosmological ether:Einstein took great pains to show that there is no such thing as motion relative to "space" - there was only motion relative to different inertial frames of reference.

The treatment of recession velocity of a galaxy as something fundamentally different than motion relative to an inertial frame of reference is never explained. Is it any wonder so many cosmologists are "confused"?

AM

Andrew Mason

Cosmological theories of superluminal expansion of space etc. are interesting but one should remain skeptical. The authors of that Scientific American article (Lineweaver and Davis) seem to think that there is a right and wrong explanation. This in itself makes be very suspicious of their claims.

In science there are only two kinds of explanations: wrong ones and possibly right ones. There are explanations that conflict with proven facts. These are wrong explanations. There are explanations those that do not yet conflict with proven facts. These are possibly right.

Lineweaver and Davis identify "wrong" explanations not because such explanations conflict with any facts. Rather they identify them as "wrong" because they conflict with other explanations that are possibly right (eg. the theory of the expansion of space and superluminal speeds).

The author of this paper does not seem to agree that superluminal speeds are needed to explain cosmological red shift and other phenomena:

AM

Dan Frank Kue

Great thread! I'm still reviewing all the links and thoughts on this page...

IF all this is true, and the universe is NOT 27.4 billion light years across (say, 13.7 billion light years to the left and 13.7 billion light years to the right)

THEN, those distant, very young galaxies must actually be very close together, moving away from us, but also closer to each other...

SO the Universe isn't exactly expanding; those distant galaxies are moving (at near the speed of light) TOWARD that singularity, the Big Bang.

Galaxies apparently 26.4 billion light years apart are actually less than a billion light years apart and CONVERGING, not expanding.

This is the part where my brain starts trickling out my ears, which is nice because then it won't explode. Is there somewhere to get this cleared up?