The Naked Scientists

The Naked Scientists Forum

Author Topic: How do they know the Universe is still expanding?  (Read 2288 times)

Offline echoic

  • First timers
  • *
  • Posts: 3
    • View Profile
If we're seeing distant galaxies as they were billions of years ago, of course there would be a redshift, because of the Big Bang. How are they so sure the Universe is STILL expanding? Even if it has settled, we wouldn't know for ages, right?

Thanks!
« Last Edit: 05/09/2013 01:03:17 by echoic »


 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Re: How do they know the Universe is still expanding?
« Reply #1 on: 04/09/2013 20:10:41 »
The evidence for expansion (and acceleration) is the redshift, as you know.  We're pretty confident of that because we have good ways of measuring the distance of galaxies (type 1a supernovae being the common example used).  We can then look at the red shift of the spectrum of hydrogen, for example, to figure out how fast these galaxies are moving away from us (or were moving away when they emitted the light).  So far, these measurements are consistent with an accelerating expansion.  We can work out what we should see when we look out at the sky if the universe's expansion is accelerating, steady state or decelerating and so far the data is quite consistent with accelerating expansion.

How do we know it hasn't stopped since the light left these distant stars on its way to us?  We don't, but we don't know of a good physical mechanism that would stop these distant galaxies from moving away from us at an accelerating rate.  It's always possible that some new force or physical law might have come into play or that there's some sort of "wall" beyond the visible universe, but there's no evidence that this is the case, so we have no reason to suspect it's true.  Physics usually looks for the simplest explanation in terms of observed evidence and based upon current measurements, that's accelerating expansion.
 

Offline echoic

  • First timers
  • *
  • Posts: 3
    • View Profile
Re: How do they know the Universe is still expanding?
« Reply #2 on: 05/09/2013 00:03:43 »
Ah, I getcha. Thanks for the reply!

I've read that "Dark Energy" could be the explanation for the accelerated expansion. Couldn't we just be seeing red shift of the Big Bang itself, in which everything sped up until reaching a constant? Or would that change in velocity have been near instant?
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Re: How do they know the Universe is still expanding?
« Reply #3 on: 05/09/2013 02:48:29 »
The thing is that even though light from distant galaxies was emitted long ago, we assume the behavior of the universe is governed by universal laws that have held since the earliest moments of its existence.  These laws predict the red shift values that would happen if the universe was accelerating, if it were to stabilize in a constant state or if it were to decelerate and come back in a "big crunch."  The values we measure coincide very well with accelerating expansion.

One possibility is that our theories are wrong.   They're based on general relativity, which seems to be correct for things on smaller scales (solar system scale), and since we expect laws to scale up, we assume it holds for galaxies and even the whole universe.  If it weren't to hold, some other law might come in and explain our measurements without need for acceleration.  So far, there hasn't been a very plausible replacement for general relativity.  So most scientists are left thinking that general relativity is correct and the measurements are also correct--so the universe has an accelerating expansion.

What could cause this?  Well, the sum of all matter and energy in the universe should determine this in general relativity.  For an accelerating expansion, we need enough energy to push the universe apart at an accelerating rate.  The problem is, when we look through our telescopes we can't find this energy.  So we call it "dark energy."  It might be a real thing, or, as I mentioned above, general relativity might be wrong.  But until someone comes up with a plausible alternative to general relativity, most scientists go with the easy explanation that there's something out there that we haven't seen yet.  It's particularly plausible because we know plenty of things we wouldn't expect to see already (vacuum energy, neutrinos,...).  None of the ones we've managed to find so far account for this, but it means that lots of things already exist that can't be spotted with telescopes, so it wouldn't be too surprising if there was one more, "dark energy."
 

Offline echoic

  • First timers
  • *
  • Posts: 3
    • View Profile
Re: How do they know the Universe is still expanding?
« Reply #4 on: 05/09/2013 05:41:38 »
Awesome,  thanks JP! I guess all we can do is trust their methods of calculation are correct for now.
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Re: How do they know the Universe is still expanding?
« Reply #5 on: 05/09/2013 18:45:03 »
Awesome,  thanks JP! I guess all we can do is trust their methods of calculation are correct for now.

Or if you're well-versed in math, you can try to work out a better theory.  But that's beyond me (and I've got a Ph.D. in physics)!  It takes a lot of very specialized training in general relativity itself to really understand the theory enough to try to replace it. 
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Re: How do they know the Universe is still expanding?
« Reply #6 on: 08/09/2013 01:54:54 »
Quote from: echoic
I've read that "Dark Energy" could be the explanation for the accelerated expansion.
That's true by definition. The term dark energy is the name given to what matter is causing the universe to undergo accelerating gravitational expansion. I.e.
http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/
Quote
Theorists still don't know what the correct explanation is, but they have given the solution a name. It is called dark energy.

Quote from: echoic
Couldn't we just be seeing red shift of the Big Bang itself, in which everything sped up until reaching a constant? Or would that change in velocity have been near instant?
Our knowledge and beliefs of the universe are based on theory, observation and a few educated guesses. The theory of most importance is Einstein's general theory of relativity. The observations are the 3K background radiation and cosmologtical redshift etc. one of the most important assumptions is called The Cosmological Principle which states that the universe is homogeneous and isotropic. Physical laws are assumed to remain the same with time. I.e. if Einstein's theory is correct today then it will be correct tomorrow, the day after, in 10 billion years etc.

With all this in mind we can make predictions about what to see and from that deduce what was true billions of years ago as well as what is true today. It all works out pretty well. If it didn't then observations would show us a very different universe since what we thought would have turned out to to actually be false and the portions of the universe nearby wouldn't be behaving as we predicted and that would be very bizzare indeed.
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Re: How do they know the Universe is still expanding?
« Reply #7 on: 08/09/2013 01:58:17 »
Awesome,  thanks JP! I guess all we can do is trust their methods of calculation are correct for now.
This isn't about trusting calculations. It's about trusting that nature is not consciously working to decieve us. Nobody in science ever worries that the math of a theory is wrong. One single scientist or a few who've read their work can make mistakes. Thousands upon thousands of then never do so. Worrying about scientists making mistakes in math is like worrying about all the worlds linguists making a mistake in language when ordering dinner.
 

Offline jeffreyH

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 3918
  • Thanked: 53 times
  • The graviton sucks
    • View Profile
Re: How do they know the Universe is still expanding?
« Reply #8 on: 13/09/2013 03:09:13 »
With expansion we should consider the hubble sphere and whether it can ever be reached by any object in an expanding universe. It may be that the sphere is always to all intents and purposes at infinity.
 

The Naked Scientists Forum

Re: How do they know the Universe is still expanding?
« Reply #8 on: 13/09/2013 03:09:13 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums
 
Login
Login with username, password and session length