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Copernicus
2014-Aug-27, 04:01 PM
I was wondering if there was enough visible matter in the universe to account for the critical density of the universe or does one need dark matter and dark energy to reach the critical density?

Shaula
2014-Aug-27, 05:25 PM
There is nowhere near enough visible matter to get close to the critical density. Even inferred dark matter and observed/inferred baryonic matter only get to about a third of the critical density. Adding in dark energy is believed to get you to the critical density.

WayneFrancis
2014-Aug-28, 05:31 AM
This has been a problem for a while. First there was dark matter which got the critical density to about .2 - .3 depending on the model. That was a problem for cosmologists. But when dark energy was discovered that fit very well bring it to 1.
Alan Guth goes into this in a open courseware cosmology course he taught at MIT pretty well.

Cougar
2014-Aug-28, 03:02 PM
First there was dark matter which got the critical density to about .2 - .3 depending on the model. That was a problem for cosmologists.

According to this paper by Bahcall (http://arxiv.org/pdf/astro-ph/9612046.pdf) in 1996 [pdf], the mass density of the Universe was still unknown. As you say, all the mass known or inferred to exist was way short of the critical density. There are plenty of problems for cosmologists, but AFAIK, at that time there was no reason to think that the Universe had to be at the critical density. Apparently it was not until year 2000 that the balloon-borne experiment BOOMERANG accurately measured (http://news.bbc.co.uk/2/hi/science/nature/729750.stm) the size of the largest variations in the CMB. With that known size of the largest CMB variations, using the reasoning expressed in this old BAUT thread, (http://cosmoquest.org/forum/archive/index.php/t-138023.html) it is concluded that the Universe is close to flat, so the mass density must be very close to the critical density. From this, it is inferred that the mass equivalent of the dark energy must make up the difference.

Jeff Root
2014-Aug-28, 05:22 PM
A friend asked me about 25 or 30 years ago whether I thought
the density of the Universe was exactly equal to the critical
density, and I answered that I did. It isn't something I'm at
all certain about, but I've never had a reason to change my
expectation. Clues that it is exactly the critical density have
been known for a long time.

-- Jeff, in Minneapolis

Copernicus
2014-Aug-28, 10:10 PM
Thanks for the answers. I was wondering if someone were on a ship and the ship is 1000000 kg on earth. They start moving 0.99c, near the speed of light. What would their mass look like to us? What would their mass look like to them?

Strange
2014-Aug-28, 10:26 PM
They start moving 0.99c, near the speed of light. What would their mass look like to us?

7 times greater.
http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/tdil.html#c3


What would their mass look like to them?

No different. (They aren't moving with respect to themselves!)

Copernicus
2014-Aug-28, 11:11 PM
7 times greater.
http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/tdil.html#c3



No different. (They aren't moving with respect to themselves!)

This is my conclusion as well. Not that great.

Can we tell that our galaxy is flattened from spinning?

Jens
2014-Aug-28, 11:13 PM
. Clues that it is exactly the critical density have
been known for a long time.


Yes, it seems clear that the critical density is 1 or extremely close to 1. I think one explanation of why this is so is the weak anthropic principle, but I think another possibility is that there is a deeper reason, something about the structure of the universe, that we don't understand.

Jeff Root
2014-Aug-29, 12:08 AM
Can we tell that our galaxy is flattened from spinning?
As best *I* can tell from what I've read, spiral and lenticular
galaxies are flattened for the same reason the planets orbit in
(pretty much) the same plane: Gas molecules moving randomly
in a roughly spherical volume collided with each other, with a
net loss of speed and angular momentum relative to the center
of the volume, making it collapse to a disk. As the gas further
collapsed into stars, the galaxy-scale collapse stopped because
the gas molecules were no longer colliding.

-- Jeff, in Minneapolis

WayneFrancis
2014-Aug-29, 12:12 AM
According to this paper by Bahcall (http://arxiv.org/pdf/astro-ph/9612046.pdf) in 1996 [pdf], the mass density of the Universe was still unknown. As you say, all the mass known or inferred to exist was way short of the critical density. There are plenty of problems for cosmologists, but AFAIK, at that time there was no reason to think that the Universe had to be at the critical density. Apparently it was not until year 2000 that the balloon-borne experiment BOOMERANG accurately measured (http://news.bbc.co.uk/2/hi/science/nature/729750.stm) the size of the largest variations in the CMB. With that known size of the largest CMB variations, using the reasoning expressed in this old BAUT thread, (http://cosmoquest.org/forum/archive/index.php/t-138023.html) it is concluded that the Universe is close to flat, so the mass density must be very close to the critical density. From this, it is inferred that the mass equivalent of the dark energy must make up the difference.

Like I said go see Alan Guth's explanation much better then my 1 line summary but your explanation explains it well. :)

NoChoice
2014-Aug-29, 01:20 AM
But when dark energy was discovered that fit very well bring it to 1.

Really? Dark energy has been discovered?
I must have missed that paper.
Could you please refer me to the paper where this discovery was announced?

Noclevername
2014-Aug-29, 01:49 AM
But when dark energy was discovered that fit very well bring it to 1.

Right now the existence of dark energy is just inferred. We don't know if it really exists.

Jeff Root
2014-Aug-29, 01:56 AM
It has been pretty widely accepted for over a decade now
that something which is referred to as "dark energy" exists.
It isn't certain by any means, but the astronomers who
have looked at the evidence generally think it is real, not
spurious. Whatever it is, it was discovered in 1998.

-- Jeff, in Minneapolis

NoChoice
2014-Aug-29, 02:01 AM
It has been pretty widely accepted for over a decade now
that something which is referred to as "dark energy" exists.
It isn't certain by any means, but the astronomers who
have looked at the evidence generally think it is real, not
spurious. Whatever it is, it was discovered in 1998.

It may have been postulated or invented around that time but it has not been discovered.
It's just one the many crutches we need to invent to keep our Cosmology stumbling along...

Shaula
2014-Aug-29, 03:31 AM
It may have been postulated or invented around that time but it has not been discovered.
It's just one the many crutches we need to invent to keep our Cosmology stumbling along...
Just like third generation quarks, the Higgs boson and the charm quark needed for the GIM mechanism were more invented crutches required to keep our standard models of particle physics working...

Copernicus
2014-Aug-29, 03:43 AM
I don't see a reason to doubt the possibility of dark matter and dark energy. There are so many things we can barely detect. But they are there. It could be called numerology, but it is numerology with a cause. There is certainly the evidence that we can't see something in galaxies to obtain the velocity curve. Of course the evidence for dark energy is much weaker, but something has to make up the difference of the critical density.

WayneFrancis
2014-Aug-29, 06:39 AM
Really? Dark energy has been discovered?
I must have missed that paper.
Could you please refer me to the paper where this discovery was announced?

*sigh* Ok when we discovered the evidence that the rate of the expansion of the universe is expanding and labeled that as "dark energy" Sheesh honestly. Like I said ... go view Alan Guth's courses. I made a one liner...sorry I didn't accurately boil down dozens of hours of lectures into one line properly.

Noclevername
2014-Aug-29, 08:16 AM
*sigh* Ok when we discovered the evidence that the rate of the expansion of the universe is expanding and labeled that as "dark energy" Sheesh honestly. Like I said ... go view Alan Guth's courses. I made a one liner...sorry I didn't accurately boil down dozens of hours of lectures into one line properly.

Can you provide a link?

Cougar
2014-Aug-29, 10:50 AM
It has been pretty widely accepted for over a decade now
that something which is referred to as "dark energy" exists.


Adam Riess shared both the 2006 Shaw Prize in Astronomy (http://en.wikipedia.org/wiki/Shaw_Prize) and the 2011 Nobel Prize in Physics (http://en.wikipedia.org/wiki/Nobel_Prize_in_Physics) with Saul Perlmutter (http://en.wikipedia.org/wiki/Saul_Perlmutter) and Brian P. Schmidt (http://en.wikipedia.org/wiki/Brian_P._Schmidt) for providing evidence that the expansion of the universe is accelerating (http://en.wikipedia.org/wiki/Accelerating_universe).




The Nobel committee is pretty careful to hand out Nobel prizes for actual evidence. It's too bad Nochoice isn't a little more careful with his off-hand comments.

Strange
2014-Aug-29, 11:12 AM
It's just one the many crutches we need to invent to keep our Cosmology stumbling along...

Still haven't got your head round the concept of "hypothesis" yet?

WayneFrancis
2014-Sep-01, 01:17 AM
Can you provide a link?

Sure
http://ocw.mit.edu/courses/physics/8-286-the-early-universe-fall-2013/index.htm
Full disclosure Alan Guth doesn't give EVERY lecture :)