Age of Universe: Seeing More than 13.5 Billion Yrs

[marcus]

LOL...nice analogy!

... I am assuming that density is for "today's" universe...

that's right! Definitely today's space and stars
an instantaneous spatial slice

our idea of what the universe is like right now (May 2007) is based on what we are able to see (by astronomical observation) that it has been like in the past

we can't see the present-day universe, only infer it.
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This density in an expanding universe should increase the farther we look correct?

that is right in the sense that the farther out we look the farther back into the past we get a picture of. and farther back in the past the density was higher.

the density in an instantaneous spatial slice is uniform or nearly so once it is averaged out
but what we see when we look out into the layers of past with a telescope is a mix of all the past history of the universe, so it has density changing with depth---different density in each layer

like some elaborate dessert prepared by a master chef just for astronomers.
the manifestly visible sky because of that combination of space and time is really pretty complicated---more complex than the inferred homogeneous spatial slice that we can reconstruct.

have to go, back later

 

[pervect]

The density at any instant of cosmological time should be constant, by the cosmological principle that the universe is homogeneous and isotropic.

When we look, of course, because of lightspeed delays we are seeing an earlier universe.

We expect that ever since the inflationary phase, the universe has been close to the critical density, thus

<br /> \Omega = \frac{8 \pi G}{3 H^2} \rho \approx 1<br />

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

H, the Hubble constant, varies with time (cosomological time), so density (\rho) does as well.

 

[BoomBoom]

that's right! Definitely today's space and stars
an instantaneous spatial slice

our idea of what the universe is like right now (May 2007) is based on what we are able to see (by astronomical observation) that it has been like in the past

we can't see the present-day universe, only infer it.
===============

Ok, I was just reading something and the proverbial "light bulb" went off in my head. I think I FINALLY get it. I believe where I was hung up was thinking of light years as being distance and I couldn't see how we could look in any direction and see the same universe when it took so much time for the light from those "distant" galaxies to reach us. So the deep views I referred to in my opening post that I presumed were 24 Glys apart, were actually only 3Glys apart 12 billion years ago.

In my search to find a "map" of the universe I ran across this http://people.cornell.edu/pages/jag8/spacetxt.html" and it was the best explanation I have come across. This I would recommend to any other "laymen" out there having troubles with grasping the concept as I was.

 

[marcus]

OK BoomBoom!

Now for anyone who likes to use the Google calculator, here is something fun to do with Google. You see what Pervect has just written here

\Omega = \frac{8 \pi G}{3 H^2} \rho \approx 1
...

He is not talking about the energy density, but rather the mass density. But we can stick a c^2 into the formula and change mass to energy.

Try making Google evaluate 3*c^2*H^2/(8*pi*G)
which Pervect is telling you somewhat indirectly is an approximate formula for the energy density of the universe! Very near anyway, see his approx=1 sign.

We can test if this amounts to something like 0.85 joule per cubic kilometer, because then if you multiply it by (km)^3 you should get 0.85 joules!

The formula for H, the Hubble-parameter is this:
71*km/s/Mpc (it says the expansion rate is 71 km per second per Megaparsec)

So you can plug this into the search box at Google:

(km^3)*3*c^2*(71*km/s/Mpc)^2/(8*pi*G)

You can just copy and paste it into the Google window. When I do that, and press "search", what I get back is

((km^3) * 3 * (c^2) * ((71 * ((km / s) / Mpc))^2)) / (8 * pi * G) = 0.851170439 joules[/color]

that means the critical energy density of the universe (which it would have to have exactly in order to be perfectly spatial flat) is 0.85 joule per cubic km.

And since we observe that it is very NEARLY spatial flat, we infer that the true energy density is very CLOSE to 0.85 joule per cubic km.

BoomBoom, that is why I told you earlier that the density is around 1 joule per cubic km, lifting a textbook 4 inches-----I didnt want to cut it too fine and say 0.85

 

[BoomBoom]

You can tell the mass of a spiral galaxy by how fast the edges are whirling around, which you can tell by doppler, if you can see it edge on. part of the edge is coming towards and part is going away and the dopplershift tells the speed.

This statement reminded me of something, although it is somewhat unrelated. I was watching a TV program on the Science channel, and since it was from TV, I'm not sure how accurate some of the statements they made are. Please let me know if any of the below statements are inaccurate:

1. so far to date, every galaxy examined has a supermassive black hole at the center.
2. The mass of this black hole correlates with the mass of it's galaxy (I believe they said 0.5%)
3. The speed of revolving stars on the outer edges of the galaxy correlate with the mass of the black hole.

I found this curious because I had always thought that as a black hole devours matter around it, it gains mass. If that is the case, shouldn't we see the mass of galaxies get smaller as the black hole consumes matter and therefore see the ratio of mass (BH/Galaxy) smaller the further out we look and larger as we look closer to home?