Expanding Universe

Thread Starter

rahulpsharma

Joined Sep 5, 2010
21
It is often quoted that Edwin Hubble's observations lead to the first proof that universe is expanding...!!

Then again I read that in 2011 Nobel prize was given to three physicists for observing that universe is expanding...!!

So my question is what is the difference between the work of Edwin Hubble and 2011 Nobel Laureates...

Thanks and Regards
Rahul
 

BobTPH

Joined Jun 5, 2013
2,047
No, they are very different discoveries.

Originally it was thought that the universe was expanding, but at a decreasing rate. This must be true if the only force operating at cosmological scale is gravity, i.e. the theory of general relativity. In fact, Einstein was disturbed that his new theory predicted that the universe must be expanding, when it was thought that it was in a steady state.

Hubble discovered that it was indeed expanding.

The newer discovery is that the rate of expansion is increasing. This cannot happen if the only force is gravity, it must expand at a decreasing rate since gravity is always slowing the expansion. The fact that the rate is increasing means that there is a repulsive force (anti-gravity) at work. This is exactly what disturbed Einstein when he thought it was steady state.

Bob
 

Thread Starter

rahulpsharma

Joined Sep 5, 2010
21
Thank you nsaspook and BobTPH...!!

So can I understand it as follows:

Hubble observed that Universe was expanding and his observations seemed to fit well then with General Relativity and subsequent theories of Friedmann, Lemaitre etc...!! This theory was purely about Gravity (matter)... No mention on anti-gravity (or repulsion due to Dark Matter etc)

But 2011 Nobel Laureates, went a step further and observed that universe is not JUST expanding but it is expanding at an accelerated rate, which cannot be explained by the theory of matter (gravity) alone... And this is where Dark Matter enters the scene...!!

Is the above lay interpretation in order...??

Thanks and regards
Rahul
 

nsaspook

Joined Aug 27, 2009
6,472
It's the other 'Dark' force here, not Dark Matter.
https://en.wikipedia.org/wiki/Dark_energy
In physical cosmology and astronomy, dark energy is an unknown form of energy which is hypothesized to permeate all of space, tending to accelerate the expansion of the universe.[1][2] Dark energy is the most accepted hypothesis to explain the observations since the 1990s indicating that the universe is expanding at an accelerating rate.[3]
...
Dark energy's status as a hypothetical force with unknown properties makes it a very active target of research. The problem is attacked from a great variety of angles, such as modifying the prevailing theory of gravity (general relativity), attempting to pin down the properties of dark energy, and finding alternative ways to explain the observational data.
 
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sagor

Joined Mar 10, 2019
115
There is dark matter and dark energy. One is being "converted" to the other somehow (I don't recall which to which). This then allows the universe to increase the speed at which it expands. Eventually, all galaxies will be moving away from earth so fast that we will never see the visible light from them any more. Thus, eons from now (billions of years??), our sky will no longer have any stars at night, just blackness (assuming earth is not consumed by the sun by then).
My question is, what happens when expansion reached the speed of light?? Will it not be limited in acceleration or will the laws of physics be broken?
 

nsaspook

Joined Aug 27, 2009
6,472
There is dark matter and dark energy. One is being "converted" to the other somehow (I don't recall which to which). This then allows the universe to increase the speed at which it expands. Eventually, all galaxies will be moving away from earth so fast that we will never see the visible light from them any more. Thus, eons from now (billions of years??), our sky will no longer have any stars at night, just blackness (assuming earth is not consumed by the sun by then).
My question is, what happens when expansion reached the speed of light?? Will it not be limited in acceleration or will the laws of physics be broken?
Space expanding at FTL speeds doesn't break causality.

 

jpanhalt

Joined Jan 18, 2008
8,059
Space expanding at FTL speeds doesn't break causality.

Now that is interesting. Next time I am ticketed for going 67 in a 60 mph zone, I wonder whether that could be used as an affirmative defense, "Officer, I wasn't going 67 mph. I was only going 55 mph. The space between me and your radar was contracting faster."
 

nsaspook

Joined Aug 27, 2009
6,472
Now that is interesting. Next time I am ticketed for going 67 in a 60 mph zone, I wonder whether that could be used as an affirmative defense, "Officer, I wasn't going 67 mph. I was only going 55 mph. The space between me and your radar was contracting faster."
The only way that would work is if the officer could never reach you to give you a ticket from a radar return that never existed.
 
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profbuxton

Joined Feb 21, 2014
418
Not entirely convinced about dark matter and energy. Seems too much like another form of the Aether. And its just been thrown into the equation to make things seem "tidy".
If the universe is expanding at an accelerating rate why are the galaxies (including ours) still hanging together. The should be spreading out all over space surely.
Nobody really knows whats going on, maybe its little invisible fairys pushing us around, Thats as good as dark matter and energy.
I suspect that much more knowledge is needed to sort this out but I will be very surprised if it turns out to be anything "dark".
 
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