On the Expansion of the Universe and such things

[vexx32]

I am familiar with the often repeated idea that the universe is expanding, and the expansion appears to accelerate when looking at more distant objects. However, my question is on the logic of this idea.

From what I have learned and what I have been able to find, when discussing this particular thing, all observations are stated in present tense. The objects in the distance are moving away from us. Logically, this makes sense on the surface. We've observed it, and as such it makes sense to say it is happening now.

However, that is not the case. Due to the fact that light only travels so fast, we cannot see the present-day state of the whole universe. The farther out we look, the more ancient the images of stars we see. The light from recent events has not had time to reach us, so we see light from many thousands and sometimes millions of years ago.

My question is thus: does it make logical sense to say that the universe is expanding at all? If the evidence for expansion of the universe comes (effectively) from looking at images of the past -- that is what one is doing when stargazing upon distant objects -- does it make any sense whatsoever to claim that the universe is currently expanding. As far as I can see (mind, I'm no astrophysicist or anything of the sort), the only conclusions we can possibly make about the expansion of the universe are about what happened then. We cannot see what is happening now.

And since the further out you look, the further back into the past you look, it only makes sense that the stars from further away are travelling away faster than those closer, simply because the light we see from them is showing us an older state of the object in question, meaning that the further out you look, the closer you get to the time of the Big Bang. The closer you are to the time of the Big Bang, the faster the objects would appear to be moving, still being flung outwards by the energy of the Big Bang. If as you look closer to the present (i.e., at objects closer to us) the expansion seems to slow, is it not then logical to think that the expansion is not in fact speeding up, as a great deal of scientists claim, but in fact slowing down?

I mean, yes I expect them to take account for the time difference, but nowhere have I ever seen evidence that they take into account the fact that the distant and faster-moving objects are being seen from further in the past.

Feel free to use any amount of math necessary (if any) to explain why my logic is flawed (if indeed it is) or what bit of information I'm missing that would make sense of this apparent oversight of the astronomers. I've been looking for an answer for this for a while, and I haven't found one yet.

[Stochasty]

It should be noted that there are, as of yet, no direct observations of change in the rate of recession for specific objects in the distant Universe - we simply haven't been observing long enough with the type of precision needed to make those types of direct observations (although I've heard a couple of people in the field tell me that we're getting close to the ability to do this).

Rather, our inference that the Universe is accelerating is by looking at luminosity versus redshift of various standard candles - something known as the distance ladder - in order to get an idea of how far each object in the Universe is away from us (and therefore how old the light coming from that object is) and how fast that object is receding from us. Once we have that information for enough points over the specific range of distances we're interested in, we can curve fit to get an idea of how fast the Universe was expanding at each epoch. This is entirely the basis for the claim that the Universe is accelerating.

A quick arXiv search turned up this short review article by Robert Caldwell. It's a little bit on the technical side, so you might have trouble following all of the math, but it should give you an idea of the evidence used to establish cosmic acceleration. If you have questions about the details, I can probably answer them.

[vexx32]

Excellent, thankyou. I shall peruse it tomorrow Looks about right for what I'm after

[Spacial_Anomaly]

From the first Friedmann equation we know

da/dt=aH_0*sqrt(Omega_m*a^-3+Omega_Lambda)

ignoring all Omegas but the Omega_Lambda (The Omegas are very well known, especially with the recent announcement of PLANCK data, Omega_Lambda~0.73, Omega_m~0.27) we get

da/dt=aH_0*sqrt(Omega_Lambda)

-> a ~ exp(H_0*sqrt(Omega_Lambda*t) (De-Sitter-space)

So if a grows da/dt also grows -> acceleration.

P.S. Does this Forum have LaTeX support?

[Stochasty]

Sadly, no LaTeX support that I'm aware of, Spacial.

One thing that is important to point out here for vexx is that cosmologists tend not to work in coordinate time, but rather in redshift (since redshift is what we have the ability to measure directly).

[vexx32]

Yeah, I'm aware of that, but the redshift we can see... does it reflect the fact that what we're seeing is not what's happening at present, or are we determining that in a different manner. I suppose that's a shorter form of my question.

[Stochasty]

The answer is yes, but the details as to why are a bit technical. If you dig into the math in section 2.1, you'll see that the equations governing the history of expansion are reparameterized into things we have the ability to measure today - redshift z, the present Hubble rate H_0, and the present deceleration parameter q_0 (I should note that although it's called the "deceleration" parameter, it's value is negative, indicating a present acceleration of the Universe).

[vexx32]

Alright, I'll have a read of it and come back if I need you to explain it in more detail. Thankee

[Stochasty]

ignoring all Omegas but the Omega_Lambda (The Omegas are very well known, especially with the recent announcement of PLANCK data, Omega_Lambda~0.73, Omega_m~0.27) we get

da/dt=aH_0*sqrt(Omega_Lambda)

-> a ~ exp(H_0*sqrt(Omega_Lambda*t) (De-Sitter-space)

So if a grows da/dt also grows -> acceleration.

This is the \lambda\CDM model, which is assuming the answer (ie, assuming that dark energy is a cosmological constant). If you want evidence as to why the \lambda\CDM model might be valid, you have to dig into the luminosity vs. redshift curves, early Universe structure formation, etc.