# Universe always expands???

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This doesn't make sense for me; "At the present rate, the universe would expand forever while gradually slowing, but never stopping." how would that be? I know the Universe is Infinite but for some reason that just stuck out.

Well, you see, what he means is that the Universe is continually expanding (since the Big Bang), but it will continue to slow down GRADUALLY. Very slowly, to infinitely small numbers.

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Recent results show that $\Omega$ is at least $0.3$. That figure includes the mysterious dark matter that lurks invisibly in space and has yet to be fully understood.

Hmm, which $\Omega$ is that? NASAs latest WMAP data concludes that $0.9988\leq\Omega_{Tot}\leq1.0116$. Before that, earlier WMAP/SDSS/etc. indicated that it was also around $1$.

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The whole deal with finding the value for omega was because inflation predicts that the value for omega should be one. Determining this was proof for some inflationary universe models. The discovery of Dark Energy ten years ago boosted the value up to nearly one.

Also, Dark matter, the universe is not infinite. For the universe to be infinite it must have expanded at an infinite rate for a short time or at a finite rate for an infinite time, neither of which is the case.

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Well, you see, what he means is that the Universe is continually expanding (since the Big Bang), but it will continue to slow down GRADUALLY. Very slowly, to infinitely small numbers.

Ok I get it, the numbers are infinitely small.

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the universe is not infinite

It might be spatially infinite, we simply don't know yet, hence the value associated with $\Omega$. Less than 1, would be infinite.

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The value for omega determines the future of the universe:

http://en.wikipedia.org/wiki/Ultimate_fate_of_the_universe

An open universe could expand theoretically forever to an infinite size. However, this value of omega does not mean that the universe is currently infinite in size.

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Doh, not sure what I was thinking when I posted that (I've got tinnitus at the moment...excuses excuses.)

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Not a problem. We're even.

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Are we not mixing up the definitions here?

Typically, $\Omega$ stands for the curvature of the universe. This $\Omega$ is measured to be close to 1, which means a flat universe.

The cosmological constant on the other hand, is more commonly denoted $\Lambda$ or when connected to the calculation of the curvature of the universe $\Omega_{\Lambda}$ is used.

Now... $\Omega_{\Lambda}$, or $\Lambda$, describes the "destiny" of our universe (big freeze, big crunch or just even out), and as I read from WMAP data it is around $\Omega_{\Lambda}\approx0.75$ atm. The "real/standard cosmological constant $\Lambda$," is it then $\Lambda=1-\Omega_{\Lambda}\approx0.25$?

It is a mess to read the WMAP articles, since I'm just a layman in the area.. and when I took a couple of astronomy/cosmology courses 10+ yrs ago, dark matter wasn't even know.

Edited by Dr. Jekyll
multiple post merged
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I'm very much the layman in this area, I won't be covering this formally for another few years, though I'm very interested in the subject all the same, despite my lack of technical knowledge...hopefully Martin will swing by and clear up any confusion.

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Are we not mixing up the definitions here?

You mean in this thread, just recently? I don't think so. Arch and Snail seem to be communicating. Almost everybody seems to realize that conventional Omega is the ratio of density to critical density. So if Omega >1 then density is more than critical (spatial closure).

A spatially closed universe, with finite spatial volume, can still expand indefinitely. It does not necessarily experience a Crunch. Can keep expanding due to the effect of dark energy

Typically, $\Omega$ stands for the curvature of the universe. This $\Omega$ is measured to be close to 1, which means a flat universe.

Typically Omega does not stand for curvature. It stands for a certain ratio of densities. Omega = 1 means density = critical, this causes space to be flat--which means curvature = 0. In other words Omega = 1 means that curvature is zero.

Now... $\Omega_{\Lambda}$, or $\Lambda$, describes the "destiny" of our universe (big freeze, big crunch or just even out),

Lambda by itself does not determine the destiny. Even with a positive Lambda (which is what is measured) other things can enter in.

and as I read from WMAP data it is around $\Omega_{\Lambda}\approx0.75$ atm.

That's right! Omega sub Lambda is the ratio of estimated dark energy density to critical density. (all the Omega numbers here are fractions of critical).

And a common estimate is 0.75, or sometimes 0.73. That means dark energy density is about 75 percent of critical.

The "real/standard cosmological constant $\Lambda$," is it then $\Lambda=1-\Omega_{\Lambda}\approx0.25$?

No. That is not how Lambda is defined. Lambda can be written in units of curvature (as the reciprocal of length squared, the reciprocal of area) or it can be written as an energy density---the putative dark energy density. If it is written as a density it comes to about 0.6 joule per cubic kilometer.

Lambda is a physical quantity, not a pure number. It has units. Critical density is about 0.8 joule per cubic kilometer. When you take ratios of the same type quantity you get simple pure numbers without units, like 0.75 for example. Often easier to work with.

==================

In one of your posts you gave a WMAP figure for Omega. I didn't check it but it sounded right. Some range like 0.998 to 1.011. I forget what you said exactly but it is good you are checking WMAP data and you extracted what I think is the right figure.

I'm very much the layman in this area, I won't be covering this formally for another few years, though I'm very interested in the subject all the same, despite my lack of technical knowledge...hopefully Martin will swing by and clear up any confusion.

Hi Snail, sorry to hear about the ear problem. Tintinitus can be really aggravating. It seemed to me that you all were doing fine, and collectively had a grasp.

As a first approximation at least, Arch seems to know the stuff, or to be learning it. I didn't study the thread carefully just got an overall impression.

Is there some confusion? Can you identify what is puzzling?

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Hi Snail, sorry to hear about the ear problem. Tintinitus can be really aggravating. It seemed to me that you all were doing fine, and collectively had a grasp.

As a first approximation at least, Arch seems to know the stuff, or to be learning it. I didn't study the thread carefully just got an overall impression.

Is there some confusion? Can you identify what is puzzling?

I wasn't too sure about Dr Jekyll's post, but not confident enough to comment, from what I understand of the Friedmann equations (which is nowhere near enough...*but there's some good news, which I'll explain about in a sec.) is that, as you said $\Omega$ is just the observed density over critical density, i.e $\frac {p}{p_c}$ IIRC a value for the critical density is found be setting $\Lambda$ to zero. There's the Hubble parameter $H$ in the equations which gives the rate of expansion. You can probably tell my knowledge is a little cursory, but the equations give an overall geometry of the universe...spatially flat, open et.c

*However, I started a thread a while ago, as I was concerned about the depth in to Einsteins equations (which you need before moving on to the Friedmann equations) and cosmology in general that my degree would cover, and lo and behold two courses are being introduced in 2 years time, which cater for exactly that, which is great news for me, as I'm looking to do an MSc in the subject.

EDIT: I just remembered there's a download available where you can tweak the values of the Friedmann equations, so you can get a graphical representation of what's going on. It was posted by a tutor on my Uni forums (different course), but sadly they get closed each year, but I'll try and find it nevertheless.

Edited by Snail
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You mean in this thread, just recently? I don't think so. Arch and Snail seem to be communicating. Almost everybody seems to realize that conventional Omega is the ratio of density to critical density. So if Omega >1 then density is more than critical (spatial closure).

I was aiming mainly at Air's post, who talked about Omega i relation to the fate of universe .. and then Snail filled it up (correctly) with associating it with the geometry of the universe.

For the rest, thanks for the lecture! You clear up quite a bit for me. Thanks!

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• 2 weeks later...

The universewill go on expanding until it utilizes all its energy,right....,like a star

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The universewill go on expanding until it utilizes all its energy,right....,like a star

Assuming the standard cosmology model is right, expansion is not subject to that kind of energy limitation and the universe will continue expanding indefinitely.

You might be interested in taking a look at this article on standard expansion cosmology that was published in the Scientific American. It does a good job of explaining the basics. They use it at Princeton for their astronomy course and have this copy of it online:

There is currently a strong consensus among professional astronomers favoring the standard model of the universe because it fits the data remarkably well. Competing models don't do as well. This doesn't mean it won't be challenged in the future, but for the time being it is the main picture in use. This SciAm article has often been recommended by people at SFN, a lot of people find it useful.

Anyway, to answer: expansion is expected to continue indefinitely, and to continue accelerating, because of the positive value of the cosmological constant Lambda discovered in 1998 by two teams of astronomers working independently of each other. Their measurements have been scrutinized extensively since then and further observations continue to confirm them.

It is a strange conclusion, frankly. Intuitively one might expect the expansion to "run out of gas" like a car. In our human experience all things have an end. But the standard model has been repeatedly tested and nobody has been able to come up with anything better. The model is called LambdaCDM (for the positive Lambda feature and for Cold Dark Matter) after two of its distinguishing features.

Since it is currently the best fit to the data, it is a good idea to learn about it and understand the dominant mainstream model before you go looking at alternatives. Know what the alternatives are alternative to.

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