# Calculating The Age Of The Universe

Tempest
I have a serious question which has been on my mind for awhile. I was thinking about the problem three months ago, and then forgot about it. My question is this, suppose that the theory of relativity is totally wrong ok? And so now I want to try to figure out how old the universe is, lets say in years.

Ok so, start off at the first moment in time. Lets suppose all the matter in the universe was very close to the center of mass of the universe (CMU). Ok so, there is a huge outwards motion of particles as we move towards now. Now during that expansion, gravity worked to slow it down. But of course, as matter moves further and further away from the CMU (in a roughly symmetric way) that force is approaching zero. Lets say after a few million years, its zero. So after a few million years ago, the center of mass of our solar system was moving outwards from the CMU at a constant speed, lets call it V.

V = distance/time

So now, if we know this speed, and we know the distance the center of mass of our solar system is away from the center of mass of the universe, then we can compute the age of the universe.

So thats how I want to do it.

So let T denote the age of the universe. Solving for T we have:

T = age of the universe = D/V

Where D is the distance the center of mass of our solar system is away from the center of mass of the universe, and V is the roughly constant speed at which the center of mass of our solar system is moving away from the CMU.

So here comes the question, how can we figure out in what direction we have to look, to be looking towards the center of mass of the universe.

I am sort of wondering if we could triangulate it, or already have done so.

From where we are, I am thinking there is a plane through which all our planets orbit. I know some planets are not in the plane, but I think most sort of travel in a common plane. So now, is it the case that the center of mass of the universe lies in this plane? That would at least narrow it down a bit. Then perhaps if we looked at some tilted galaxies we could triangulate. Then if we can figure out how far they are away from us, we would know our distance from the CMU. After that, it is only a means of measuring our speed away from that point, computing it in some fashion or other.

Before everyone criticizes this post, I would just like to say one thing:

I think 14 billion years is way way way not enough time for the universe to be as it is. Stars had to form, then explode, then reform. Everything had to travel really far to get where it is. Oceans had to form, rain rain, evolution, life, etc. I mean, saying that the age of the universe is of the same order of magnitude as the age of the earth, is totally unintuitive. I am thinking that the universe is over 100 billion years old, and I wouldnt even be surprised if it was a trillion years old. Beyond that I think is pushing it. I have an intuitive understanding of how long a year is, and I am just saying, 14 billion years is too young. So anyways, does anyone know what direction to look to find the CMU?

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NateTG
Homework Helper
You speak about the center of mass of the universe - a concept that only makes sense if the universe is finite and we can observe all of it.

Let's assume, for a moment, that the universe is bounded and expanding approximately uniformly, and that we can see the ends of the universe.

Then either:
1. We're (roughly) in the center of the universe.
or
2. We should see more distant locations in some particular direction.

AFAIK, there is no strong profile of distance/direction as described in 2, so we'd be the center of the universe.

I think that there's relatively strong evidence that the universe is indeed expanding approximately uniformly, so the only other possibility that we have left are that we cannot see the ends of the universe - so CMU is not a usefull concept.

In either case, the approach that you propose doesn't seem particularly promising.

chroot
Staff Emeritus
Gold Member
The observable universe is, by definition, finite, and that's all that really matters to us. Whether or not the universe is really infinite has no bearings on the fact that we can only see some part of it. (The light from more distant locations, if there are any, hasn't had time to reach us yet.)

It seems to be true that our observable universe is isotropic, and thus has no definable center of mass.

As far as 14 billion years not being enough to suit your intuition, that's just speculation. Large, hot, OB stars, the sort that probably formed first in the unvierse, have lifetimes of only tens of millions of years. There could have been thousands of generations of them by now. We understand quite a lot about how stars form, live, and die, and 14 billion years is really plenty of time. In fact, there are a lot of arguments against a much older universe; stars are constantly trying to drive the mass fraction of the universe towards Helium, for example.

- Warren

Redfern
Originally posted by NateTG
You speak about the center of mass of the universe - a concept that only makes sense if the universe is finite and we can observe all of it.

Let's assume, for a moment, that the universe is bounded and expanding approximately uniformly, and that we can see the ends of the universe.

Then either:
1. We're (roughly) in the center of the universe.
or
2. We should see more distant locations in some particular direction.

AFAIK, there is no strong profile of distance/direction as described in 2, so we'd be the center of the universe.

I think that there's relatively strong evidence that the universe is indeed expanding approximately uniformly, so the only other possibility that we have left are that we cannot see the ends of the universe - so CMU is not a usefull concept.

In either case, the approach that you propose doesn't seem particularly promising.
This is wrong. If there is no center of mass of the universe, then the amount of matter in the universe is infinite. The amount of matter in the universe isn't infinte, hence there is a place in the universe which is the center of mass. Space itself is infinite, but not the amount of matter. So this guy has a useful concept.

Regards,

Redfern

Redfern
Originally posted by chroot
The observable universe is, by definition, finite, and that's all that really matters to us. Whether or not the universe is really infinite has no bearings on the fact that we can only see some part of it. (The light from more distant locations, if there are any, hasn't had time to reach us yet.)

It seems to be true that our observable universe is isotropic, and thus has no definable center of mass.

As far as 14 billion years not being enough to suit your intuition, that's just speculation. Large, hot, OB stars, the sort that probably formed first in the unvierse, have lifetimes of only tens of millions of years. There could have been thousands of generations of them by now. We understand quite a lot about how stars form, live, and die, and 14 billion years is really plenty of time. In fact, there are a lot of arguments against a much older universe; stars are constantly trying to drive the mass fraction of the universe towards Helium, for example.

- Warren
The observable universe isn't isotropic, in fact its anything but isotropic. If you don't think there's a center of mass of the universe, you make no effort to locate one.

Regards,

Redfern

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russ_watters
Mentor
Originally posted by Redfern
This is wrong. If there is no center of mass of the universe, then the amount of matter in the universe is infinite. The amount of matter in the universe isn't infinte, hence there is a place in the universe which is the center of mass. Space itself is infinite, but not the amount of matter.
Not true. Its difficult to picture because we're only used to dealing with 3 space dimensions, but try this: where is the center of the surface of the earth?

The surface of the earth is 2d, finite, boundless, and therefore without a center. The univere could be the same way but in 3d.
The observable universe isn't isotropic, in fact its anything but isotropic. If you don't think there's a center of mass of the universe, you make no effort to locate one.
Many, many people have expended a lot of effort on this problem. If the universe really were not isotropic on the large scale, finding the center would be a piece of cake. But it is. The two best pieces of evidence are Hubble photos that show that everywhere we look we see galaxies, and the cosmological background radiation is roughly uniform in all directions.

Labguy
Originally posted by Redfern
The observable universe isn't isotropic, in fact its anything but isotropic. If you don't think there's a center of mass of the universe, you make no effort to locate one.

Regards,

Redfern
Correct; it is anisotropic according to:
http://www.astro.ucla.edu/~wright/CMB-DT.html and: http://home.fnal.gov/~scranton/LensedCMB/reference.html [Broken]
and a few hundred other sources. The variance is small but real, and we are "moving" with respect to the CMBR "sphere".

But, it seems that the original post was looking for a(an) particular, identifiable coordinate(s) for a CM of the universe, or CMU as it was put. I agree that with a finite mass there must be a center-of-mass acting gravitationally to where, if a closed universe, a Big Crunch would be drawn. With any finite amount of matter and gravity, there must be a CM, even though its "location" could, and probably is, changing with expansion and formation of any new baryonic matter.

That said, I have to agree with NateTG's post and chroot's first paragraph, especially on the points that (1): such a "location" is meaningless and of no useful value (or research \$) and (2): there would be no way of "measuring our speed away from that point" as in the original post.

But on your quoted post above, an "effort to locate one" (CMU) would for all practical purposes, and with any observation techniques imaginable, would simply be solved by the most-accepted view available that the CMU is right here at my house, and right at your house too! I'm not saying that trying should be tossed out, just that there are still too many unanswered cosmo-questions to make that of any value until more current questions are answered (confirmed?).

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Labguy
Originally posted by russ_watters
Not true. <snip>..
If the universe really were not isotropic on the large scale, finding the center would be a piece of cake. But it is. The two best pieces of evidence are Hubble photos that show that everywhere we look we see galaxies, and the cosmological background radiation is roughly uniform in all directions..<unsnip>
Galaxies everywhere for sure, but "roughly uniform" is not truly uniform, and the anisotropy is very significant to about every cosmological theory going. In fact, it is the main reason that the inflationary theories (23+) are in vogue; they explain the anisotropy being observed.

I already agreed on the "no center for the universe" part.

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Redfern
Originally posted by russ_watters
Not true. Its difficult to picture because we're only used to dealing with 3 space dimensions.
It is absolutely true, and you shouldn't be trying to picture anything else. There are exactly three spatial dimensions, as was clearly shown by the ancient greeks.

At most three infinite straight lines can pass through a single point, such that the lines are mutually perpendicular. So your thinking anything to the contrary, will place an error in your understanding of space. Next, the reason there is a center of mass of the universe, doesn't only depend upon the absolute fact that space is three dimensional, it also depends upon the amount of material being finite. So granted that

1. Space is Euclidean (i.e. 3-dimensional) and
2. The amount of matter is finite

It follows that there is a center of mass of the universe.
The above two conditions are necessary and sufficient for there to be a center of the universe.

Since space really is 3-D, and the amount of matter is finite, the universe has a center of mass. There is no need to wonder where the center of a surface is, because a surface is 2-D, and the universe is 3-D. I don't see what your point was with that.

Regards,

Redfern

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chroot
Staff Emeritus
Gold Member
Originally posted by Redfern
So granted that

1. Space is Euclidean (i.e. 3-dimensional) and
2. The amount of matter is finite

It follows that there is a center of mass of the universe.
Neither of these conditions are necessarily true, and most people believe neither are.

[mentor hat on]
Also, I'd like to you to temper your attitude a bit here on physicsforums. You're certainly not "absolutely right" because you put your bet on the side of the ancient Greeks. Many of us here seem to have a better education about the topic than you. Don't close your ears too quickly -- you might miss something. If you don't see russ's point yet, it doesn't mean he had no point -- it means you need to think about it some more.
[mentor hat off]

- Warren

chroot
Staff Emeritus
Gold Member
Originally posted by russ_watters
The analogy of the surface of the Earth follows the same rules. It is both finite and euclidean (though only 2d - it is curved in 3d)
Sorry, russ -- the notion of curvature for a manifold is well-defined even without reference to any higher-dimensional embedding. In other words, a 2-sphere does have intrinsic curvature.

- Warren

russ_watters
Mentor
Originally posted by chroot
Sorry, russ -- the notion of curvature for a manifold is well-defined even without reference to any higher-dimensional embedding. In other words, a 2-sphere does have intrinsic curvature.

- Warren
I get a little thin on that - I actually deleted my post while you were responding because it wasn't looking quite right. I kept making changes then said, ehh, screw it.

Redfern
Originally posted by chroot
Neither of these conditions are necessarily true, and most people believe neither are.

I don't want to know what you believe, I want to know what you know. Is space three dimensional yes or no?

chroot
Staff Emeritus