The age of the universe vs. the speed of light

[cisco kid]

I have recently been watching the series "Into the Universe with Stephen Hawking" and I have a question (commence eye-rolling of anyone reading this). I don't think the series is supposed to be an educational tool necessarily, its more of a horizon expanding view of some high level topics with low level explanations. At any rate, Dr. Hawking stated something that I am having a hard time wrapping my head around. I understand that the laws of physics state that there is a maximum speed limit, which we call the speed of light. This particular episode is explaining the origins of the universe via the big bang. He states "by the time the cosmos was ten minutes old, it was already thousands of light years in diameter." My question is, how could the matter travel thousands of light years in ten minutes? Wouldn't it take thousands of years, if that is the maximum speed?? Obviously I'm missing something here...

 

[mathman]

The general idea is something which goes by the name "inflation", where space expanded at an extremely high rate carrying stuff with it.

http://en.wikipedia.org/wiki/Inflation_(cosmology )

 

[Drakkith]

The key thing to understand is that nothing can travel THROUGH space at greater than light speed, aka c, however inflation and expansion of the universe happens because space itself is expanding and carrying matter and radiation with it. Thus two things that are a billion light years apart can be moving through space at 200 km/s towards each other yet because of expansion the distance between them is actually increasing over time.

 

[cisco kid]

Crazy. So I guess that is why people use the raisin bread analogy? It's impossible for the raisin to move through the dough at the speed of light, but combined with the expansion of the dough it's speed relative to another raisin would equal a speed faster than light? Something like that?

 

[Drakkith]

Crazy. So I guess that is why people use the raisin bread analogy? It's impossible for the raisin to move through the dough at the speed of light, but combined with the expansion of the dough it's speed relative to another raisin would equal a speed faster than light? Something like that?

That is exactly right!

 

[cisco kid]

Awesome! Thanks for explaining!

 

[wildwill]

but what is it expanding into?

 

[Drakkith]

but what is it expanding into?

To the best of our knowledge it is not expanding into anything. Read the FAQ here on the cosmology forum.

 

[wildwill]

then how is it expanding?

 

[Drakkith]

then how is it expanding?

Are you asking for how expansion works and effects the universe or what is the underlying cause of expansion? In either case see here: http://en.wikipedia.org/wiki/Metric_expansion_of_space

 

[wildwill]

okay then thanks i think i understand it

 

[Calimero]

The key thing to understand is that nothing can travel THROUGH space at greater than light speed, aka c, however inflation and expansion of the universe happens because space itself is expanding and carrying matter and radiation with it.

Hi Drakkith. I fail to understand your first sentence. How do you measure or define object's speed or velocity relative to the space ?!? If you mean that nothing massive can't outrun light, then say so. But nothing massive can't outrun light in expanding space also.

If someone comes here looking for an answer to a basic straightforward question, like cisco kid did, it is much better to link him to the some cosmology tutorial, like ned wright's pages, instead of trying to give him wrong answer.

 

[D H]

The key thing to understand is that nothing can travel THROUGH space at greater than light speed, aka c, however inflation and expansion of the universe happens because space itself is expanding and carrying matter and radiation with it.

Hi Drakkith. I fail to understand your first sentence. How do you measure or define object's speed or velocity relative to the space ?!? If you mean that nothing massive can't outrun light, then say so. But nothing massive can't outrun light in expanding space also.

Drakkith's answer, while perhaps a bit too terse is essentially correct.

The Davis and Lineweaver paper, http://www.mso.anu.edu.au/~charley/papers/DavisLineweaver04.pdf, describes three common misconceptions regarding the expansion of space:

#1: Recession velocities cannot exceed the speed of light.
#2: Inflation causes superluminal expansion but normal expansion of the universe does not.
#3: Galaxies with recession velocities exceeding the speed of light exist but we cannot see them.​

The paper delves into each of these misconceptions, showing how they are just that (misconceptions, that is).

 

[Calimero]

Drakkith's answer, while perhaps a bit too terse is essentially correct.

The Davis and Lineweaver paper, http://www.mso.anu.edu.au/~charley/papers/DavisLineweaver04.pdf, describes three common misconceptions regarding the expansion of space:

#1: Recession velocities cannot exceed the speed of light.
#2: Inflation causes superluminal expansion but normal expansion of the universe does not.
#3: Galaxies with recession velocities exceeding the speed of light exist but we cannot see them.​

The paper delves into each of these misconceptions, showing how they are just that (misconceptions, that is).

From the paper you linked to:

"No observer ever overtakes light beam and all observers measure light locally to travel at c"

You completely misunderstood my post. I didn't say anything about superluminal recession velocities between galaxies, not a word.

Again, what does it mean to say: nothing can travel through space at greater then light speed? How do you define speed relative to the space? With aether, or what?

Edit: DH, I feel that we shouldn't have this discussion at all. Bcrowell wrote a nice FAQ on the subject, linking to the same paper you did. It is just painful to see someone with PF mentor badge defending statements like Drakkith's #3 in this thread.

 

[Drakkith]

Hi Drakkith. I fail to understand your first sentence. How do you measure or define object's speed or velocity relative to the space ?!? If you mean that nothing massive can't outrun light, then say so. But nothing massive can't outrun light in expanding space also.

You don't measure it relative to space, as you cannot as far as I know. However it is believed that the expansion of space is responsible for the increasing distance between objects in the universe. I simply mean that if we could stop expansion of space and just see the velocities of objects due to inertia we would see that many objects billions of light years away are actually traveling towards us. Their current observed recession velocities are due to the expansion of space, not through inertia.

Edit: Actually, if you know the rate of expansion you can calculate the effect that expansion has on objects. Since expansion increases recession velocity by 73.8 ± 2.4 (km/s)/Mpc we can measure the velocity of an object, remove the effect due to expansion, and just see the velocity of that object "moving through space".

If someone comes here looking for an answer to a basic straightforward question, like cisco kid did, it is much better to link him to the some cosmology tutorial, like ned wright's pages, instead of trying to give him wrong answer.

What was incorrect about my explanation?

 

[Calimero]

You don't measure it relative to space, as you cannot as far as I know.

Right.

However it is believed that the expansion of space is responsible for the increasing distance between objects in the universe.

Debatable, but ok.

Their current observed recession velocities are due to the expansion of space, not through inertia.

Wrong. You can not distinguish between the two. Locally, Hubble flow can be viewed as pure kinematical phenomenon. Objects recede because they have an initial velocity proportional to distance.

Edit: Actually, if you know the rate of expansion you can calculate the effect that expansion has on objects. Since expansion increases recession velocity by 73.8 ± 2.4 (km/s)/Mpc we can measure the velocity of an object, remove the effect due to expansion, and just see the velocity of that object "moving through space".

Please stop talking about velocity through space.

What was incorrect about my explanation?

What was correct? Please read bcrowell's FAQ: https://www.physicsforums.com/showthread.php?t=508610
Do you see there any distinction between objects moving through space and objects moving with space as an attempt to explain superluminal recession velocities?

 

[Cosmo Novice]

I fail to see how bcromwells FAQ disagrees with Drakkiths fundamental statements.

I think what Drakkith is saying is clear and essentially correct.

Expansion and increase in the scale factor is responsible for overall expansion of the Universe regardless of individual kinematic motion.

Calimero you are claiming Drakkith post is wron but not stating why? Kind of a difficult debate for him to have.

 

[Drakkith]

Right.

Wrong. You can not distinguish between the two. Locally, Hubble flow can be viewed as pure kinematical phenomenon. Objects recede because they have an initial velocity proportional to distance.

There is no need to locally, as there is no expansion locally since we are gravitationally bound to everything within our local galaxy cluster or supercluster I believe.

Please stop talking about velocity through space.

Umm. No?

What was correct? Please read bcrowell's FAQ: https://www.physicsforums.com/showthread.php?t=508610
Do you see there any distinction between objects moving through space and objects moving with space as an attempt to explain superluminal recession velocities?

The last line of the FAQ:

For one fairly natural definition of velocity, there are galaxies we observe that are now and always have been receding from us at a velocity greater than c.[Lineweaver]

From the paper linked in the faq:

In expanding space, recession velocity keeps increasing
with distance. Beyond a certain distance, known as the
Hubble distance, it exceeds the speed of light. This is not a
violation of relativity, because recession velocity is caused
not by motion through space but by the expansion of space.

Distant galaxies also have small local velocities, but from our perspective (at large values of d) these random velocities are swamped by large recession velocities (v).

Does this prediction of faster-than-light galaxies mean
that Hubble’s law is wrong? Doesn’t Einstein’s special theory
of relativity say that nothing can have a velocity exceeding
that of light? This question has confused generations of students.
The solution is that special relativity applies only to
“normal” velocities—motion through space. The velocity in
Hubble’s law is a recession velocity caused by the expansion
of space, not a motion through space.

I'm not seeing where I'm wrong.

 

[Calimero]

Expansion is due to the inertia. It should be clear to you, but it obviously isn't. Galaxies move apart because they did it in the past. Accelerating expansion, like the one we are witnessing, is partly due to inertia, and partly due to the dark energy repulsion. In the early universe it was almost purely inertial. Neither of these are caused with expanding space. The very meaning of the notion of expanding space is not rigorously defined. It is not a law or a cause of anything, rather it is is an intuitive framework for understanding the effects of General Relativity.
It is very wrong to say that universe is expanding because space is expanding and carrying stuff along. And it is at least very misleading to say that it is ok for two objects to recede faster then light when they are moving with space, but not through space.

How come then that there is apparent superluminal motion between distant objects? Again bcrowell's FAQ. If you define distance as proper distance D, and time as cosmological time t, then FRW metric fails to put a limit on distance of validity of Hubble's law, and differential dD/dt can exceed unity. It does not violate special relativity prohibition on light speed in any way.

 

[D H]

Expansion is due to the inertia.

No, it is not.

Expansion of space is quite different from inertia. Your view is not consistent with the big bang theory. The inflationary period (the question raised in the opening post) cannot be explained in terms of inertia. Although expansion now is much attenuated compared to that in the inflationary period, even the present-day expansion of space cannot be explained in terms of inertia. The observable universe is estimated to be about 93 billion light years in diameter, yet the universe is estimated to be only about 13.75 billion years. These two figures do not jibe with one another if expansion is inertial only.

You appear to have the mistaken concept that the big bang was an explosion in space. A much better point of view is that the big bang was an explosion of space.

 

[Drakkith]

Also, I don't believe inertia could explain the increasing recession velocities of 74 (km/s)/mpc. Is that correct DH?

 

[D H]

Also, I don't believe inertia could explain the increasing recession velocities of 74 (km/s)/mpc. Is that correct DH?

It certainly doesn't explain recession for distances greater than c/H₀.

Going back to your first post in this thread (post #2),

The key thing to understand is that nothing can travel THROUGH space at greater than light speed ...

Calimero was correct to balk at your term "nothing can travel THROUGH space at greater than light speed." I think what you meant was "locally, nothing can travel faster than light". The way you said it essentially implies an ether. Not the luminiferous ether of the late 19th century, but a more modern one: The "fabric of spacetime" as an ether. That point of view will correctly raise the hackles of our science advisors who specialize in this field.

 

[Calimero]

Expansion of space is quite different from inertia.

I couldn't agree more.

Your view is not consistent with the big bang theory.

Your view is not consistent with the big bang theory.

The inflationary period (the question raised in the opening post) cannot be explained in terms of inertia.

Let's not talk about inflationary period. It can draw our attention from the physics that we know actually something about.

Although expansion now is much attenuated compared to that in the inflationary period, even the present-day expansion of space cannot be explained in terms of inertia. The observable universe is estimated to be about 93 billion light years in diameter, yet the universe is estimated to be only about 13.75 billion years. These two figures do not jibe with one another if expansion is inertial only.

DH, you are mixing Minkowksi spacetime with FRW in a bad way. Motion of galaxies is inertial. In the early universe they move only because they were doing so in the past. Look up for "tethered galaxy" problem (Harrison 1995), if you are interested.

You appear to have the mistaken concept that the big bang was an explosion in space. A much better point of view is that the big bang was an explosion of space.

Am I?

 

[DaveC426913]

You appear to have the mistaken concept that the big bang was an explosion in space.

Am I?

It would seem so, yes:

Galaxies move apart because they did it in the past. ... In the early universe it was almost purely inertial.

If they are moving (mostly) because of inertia, how are you suggesting they got this inertia in the early universe?

Note: you didn't have to wait to be asked to provide an explanation. It makes you look evasive.

 

[Drakkith]

Calimero was correct to balk at your term "nothing can travel THROUGH space at greater than light speed." I think what you meant was "locally, nothing can travel faster than light". The way you said it essentially implies an ether. Not the luminiferous ether of the late 19th century, but a more modern one: The "fabric of spacetime" as an ether. That point of view will correctly raise the hackles of our science advisors who specialize in this field.

Well, while I understand the issue, practically everything I've ever read has explained it in a similar way. I'm just an armchair scientist, not a real one!

 

[Calimero]

If they are moving (mostly) because of inertia, how are you suggesting they got this inertia in the early universe?

I don't know that. Why objects have inertia, why do they resist change in their state of motion? Do you know maybe?

If you are asking why galaxies are moving apart then answer is probably due to the cosmic inflation. If you want safe bet, then you can say due to the initial conditions.

At this point I would like to ask any of you to provide some reference, paper, or whatever, which explicitly says that galaxies are not moving apart because they were doing so in the past. I will try to do to same, just in favor of my claims.

 

[Calimero]

Quotes from wikipedia article Metric expansion of space:

Metric expansion is a key feature of Big Bang cosmology and is modeled mathematically with the FLRW metric. This model is valid in the present era only at relatively large scales (roughly the scale of galactic superclusters and above). At smaller scales matter has clumped together under the influence of gravitational attraction and these clumps do not individually expand, though they continue to recede from one another. The expansion is due partly to inertia (that is, the matter in the universe is separating because it was separating in the past) and partly to the repulsive force of dark energy, which is of a hypothetical nature, but it may be the cosmological constant. Inertia dominated the expansion in the early universe, and according to the Lambda-CDM model (ΛCDM model) the cosmological constant will dominate in the future. In the present era they contribute in roughly equal proportions.

The expansion of space is sometimes described as a force which acts to push objects apart. Though this is an accurate description of the effect of the cosmological constant, it is not an accurate picture of the phenomenon of expansion in general. For much of the universe's history the expansion has been due mainly to inertia. The matter in the very early universe was flying apart for unknown reasons (most likely as a result of cosmic inflation) and has simply continued to do so, though at an ever-decreasing rate due to the attractive effect of gravity.

 

[DaveC426913]

I don't know that. Why objects have inertia, why do they resist change in their state of motion? Do you know maybe?

No, no one is asking why they have inertia or what it is, simply how they got moving in the first place.

If you are asking why galaxies are moving apart then answer is probably due to the cosmic inflation. If you want safe bet, then you can say due to the initial conditions.

That sounds like a ballistic cause, which is what we're claiming you think - an explosion.

 

[Calimero]

Dave, I mean, really ?!? Ballistic cause? You didn't provide any reference, which I kindly asked of you. Do you ever read other people's posts or you just keep repeating your mantras? Can you at least read my #27, I even bolded it out for you, so you don't have to read it all. It is simple wikipedia article. Can you do that, please?

 

[Drakkith]

Dave, I mean, really ?!? Ballistic cause? You didn't provide any reference, which I kindly asked of you. Do you ever read other people's posts or you just keep repeating your mantras? Can you at least read my #27, I even bolded it out for you, so you don't have to read it all. It is simple wikipedia article. Can you do that, please?

So, do you disagree with the paper in the FAQ which I linked a few paragraphs from above? And enough with the attitude. It's doing you no good.

 

[Calimero]

So, do you disagree with the paper in the FAQ which I linked a few paragraphs from above? And enough with the attitude. It's doing you no good.

Ok Drakkith, I will try it once more, if you are willing to understand then fine, if not then, well, what can I do. Motion in expanding universe is inertial (in the limit I explained earlier). Nobody in the right mind disputes that. Certainly Charles Lineweaver is not disputing that. Do you think that you can get rid of inertia, make it non-existent, just because of your coordinates choice?

The problem that I am trying to point out in our discussion is interpretation of what expanding space really means. So, yes I am disagreeing very much with statements like:

The velocity in Hubble’s law is a recession velocity caused by the expansion
of space, not a motion through space.

I think that words should be chosen with much more care, because they are easily causing misconceptions.

Very same authors (Charles H. Lineweaver and Tamara M. Davis) in summary of their "Solutions to the tethered galaxy problem in an expanding universe and the observation of receding blueshifted objects" paper conclude:

We have shown that the unaccelerated
expansion of the universe has no effect on whether an
untethered galaxy approaches or recedes from us. In a
decelerating universe the galaxy approaches us, while in
an accelerating universe the galaxy recedes from us.

 

[Drakkith]

The problem that I am trying to point out in our discussion is interpretation of what expanding space really means.

Ok. So are you going to explain what it means then?

I think that words should be chosen with much more care, because they are easily causing misconceptions.

Maybe. All I know is that a lot of people who are much more knowledgeable on the subject than I am are saying the exact same thing I'm saying. You're going to have to do more than simply tell me I'm wrong to convince me.

 

[Drakkith]

Could someone clarify this:

General relativity was specifically derived
to be able to predict motion when global inertial frames were not available. Galaxies
that are receding from us superluminally are at rest locally (their peculiar velocity,
vpec = 0) and motion in their local inertial frames remains well described by special
relativity. They are in no sense catching up with photons (vpec = c). Rather, the
galaxies and the photons are both receding from us at recession velocities greater than
the speed of light.

From here: http://arxiv.org/PS_cache/astro-ph/pdf/0310/0310808v2.pdf

Does this mean that expansion is pushing both the light and the galaxy away from us? Or does GR not say what is causing this, only that it's happening or what?

 

[Calimero]

Ok. So are you going to explain what it means then?

I'll try. It is a consequence of a FRW metric. Metric is defined that way. Observers are at rest, and space between them is expanding or contracting. There is no limit on what rate space can expand. As metric evolves through time, between the two adjacent points in space there will always appear more points of space - space is expanding. Now the most important part - metric is evolving according to the presence of matter in it. Not the other way around. So, one could claim that space is expanding due to the fact that matter is receding. Hope that helps.

 

[Drakkith]

So, one could claim that space is expanding due to the fact that matter is receding. Hope that helps.

I don't see how your explanation is any different than what has already been said.