Is the universe actually expanding?

[Stu]

Bear with me on this one...

The farther away you look, the further back in time you look...

The universe seems to be expanding faster the farther away you look

That means the universe is expanding faster as you go back in time..

Does that not mean that it is actually slowing down because the closer, and therefore more recent galaxies are not accelarating as much, and locally things are more static or even moving closer towards collision...

Another way of looking at it, if the edge of the viewable universe is 13 billion light years away and, based on what we can see now, accelerating away from us, what is it actually doing now, at this moment in time????

Just the ramblings of a crazed mind but if someone could explain it would maybe help the ramblings stop!

[Matthew.Blake]

The bits that are farther away from us are farther away because they were travelling faster relative to us than the closer bits. They are not travelling faster because they are farther away.

[Stu]

So how fast are they going now? Still moving further away or have they slowed don, even though we can't see them in thier present state?

[Earl]

Do they even still exist?

[Doc]

It's actually expanding in all directions hence the different redshifts. As there is no centre of the universe there is no expansion from a certain point.

[acey]

Imagine a rubber sheet with a grid drawn on it of 1cm squares. Stretch it at a constant rate so that the edges of the squares grow by 1mm per second. Imagine there is a galaxy on each corner (and one of them is our own galaxy). If we look at a galaxy one square away from ours than after a second it has moved a millimetre further from us. If we look at a galaxy two squares away then in the same time it will move 2mm from us. In general we will find that the apparent speed of any galaxy is proportional to its distance, though the "universe" (the rubber sheet) is expanding by the same amount everywhere. The rate of expansion in this case is 1mm per cm per second. In the real universe it's the Hubble constant.

[ollypenrice]

I believe a misconception lies in this line of the post;

' The universe seems to be expanding faster the farther away you look. '

Care is needed here. The line contains an ambiguity. The distant universe is receeding from us faster than the near universe, yes. But this is simply because there is more space expanding between us and the distant universe and us and the near universe.

Twice the distance so twice the expansion so twice the increase in recession velocity.

However, this will be the case if the universe has always expanded at exactly the same rate. It does not prove that the universe was once expanding faster.

Indeed observation suggests that the universe has been expanding at a consitent rate for much of its life but has relatively recently started to accelerate. Heat death here we come!

Olly

[Stu]

Well thanks Olly and Acey, I think I finally get that. Your explanations were very clear and helpful. Olly, you are right, I was under the impression that acceleration was related to distance but now get it.

My other question still stands though, what do we think is happening in those areas which we can only see as they were long ago, still expanding away from us?

[George Jones]

Indeed observation suggests that the universe has been expanding at a consitent rate for much of its life but has relatively recently started to accelerate.

Yes, before the present era of accelerated expansion, there was a long period of "coasting", and before coasting, there was a period of deceleration.

So how fast are they going now? Still moving further away or have they slowed don, even though we can't see them in thier present state?

We can input data obtained from observations into currently favoured mathematical models of the universe, and see what these models predict.

I have attached a plot of the speeds (relative to us) of a couple of galaxies over the history of universe and into the future. The horizontal axis is time after the Big Bang in billions of years. The vertical axis is speed relative to us (in units that produce convenient plots). "Now" is 13.7 on the horizontal axis.

One galaxy always travel faster than the other galaxy, so, as Matthew.Blake noted, this faster galaxy has always been farther away than the slower galaxy.

Notice that immediately after the Big Bang, speeds decrease until they reach a minimum at about 7.5 (billion years after the Big Bang) on the the horizontal axis, and then speeds start to increase. Around 7.5 there is a period where speed changes only gradually, the coasting period of expansion at an almost constant rate.

I am going out with my family now. I hope to get back to this thread, and explain why there are three eras (decelerating expansion, constant expansion, accelerating expansion).

Galactic_Speed.bmp

[yeti monster]

I But this is simply because there is more space expanding between us and the distant universe and us and the near universe.

I know that I'm taking this out of context somewhat, but I like the notion.

Intervening space expanding, thus pushing galaxies (etc.) further apart. Cool!

[acey]

My other question still stands though, what do we think is happening in those areas which we can only see as they were long ago, still expanding away from us?

We have no way of knowing. The assumption is that those regions are pretty much like ours, so what is seen in those distant regions is pretty much what our own region was like at that era.

This situation is typical when dealing with astronomical distances. We don't know for sure that the sun hasn't disappeared a second ago - we'd only find out eight minutes later. If the Andromeda Galaxy disappeared we wouldn't know about it for more than 2 million years. In fact when we look at M31 we don't even see it at a particular moment in time: the furthest edge of the disc is about 100,000 light years further away than the nearest edge. So we're seeing it "smeared out" over a corresponding period of time. If the whole of the Andromeda Galaxy could somehow disappear in an instant, we'd start to see it 2 million years later, then it would take another 100,000 years for the whole thing to vanish from our sky.

[ollypenrice]

. If the whole of the Andromeda Galaxy could somehow disappear in an instant, we'd start to see it 2 million years later, then it would take another 100,000 years for the whole thing to vanish from our sky.

That's a nice 'take' on a familiar theme. Very good. Now you see me, now you doooooooooooooooooooon't!!

Olly

[Stu]

Thanks all for the replies. George, i'll read through yours properly later when I have more time and look forward to the next installment! After mis-understanding this for a long time, a few things are falling in to place!

Many thanks

[Dandobi]

I'm a bit of a newbie at this but what I understand is:

1) Yes it is expanding, supported by evidence of the red shift of galaxies.

2) How fast? We don't know! Why? Because we are not sure how far objects are away so you can't be sure what the Hubble constant (the rate of expansion) is. Anyone got a really big tape measure?!

Is this right or am I talking rubbish?

[George Jones]

Because we are not sure how far objects are away so you can't be sure what the Hubble constant (the rate of expansion) is. Anyone got a really big tape measure?

Methods of determining distances to astronomical objects:

PHYS134: Astro Fundamentals: The Distance Scale.

We know the current value of the Hubble constant (which is constant in space, but which varies with time) to an accuracy of better than 10% (maybe as small as about 2%).

[Stu]

Very interesting article, thanks George.

Am I right in thinking that the small uncertainty in the Hubble constant is the difference between whether the universe will continue expanding or collapse back to a singularity again?

Another point, can anyone explain with an analogy how the universe can have no centre? Would I see the same sort of expnasion no matter where I was?

[ollypenrice]

'Another point, can anyone explain with an analogy how the universe can have no centre? Would I see the same sort of expnasion no matter where I was? '

The hoary old favourite is the surface of a sphere. The sphere obviously has a centre but the surface of a perfect sphere has no point on it any different from any other point, therefore it cannot be said to have a centre.

Then the analogy develops by making the sphere into an expanding balloon onto which coins (read galaxies) have been glued. These, being glued, cannot 'move' in the conventional sense but do move apart uniformly as the balloon inflates further. The expansion, as viewed from each coin/galaxy, is the same as from any other and is in a linear relationship with distance. Double the distance, double the recession velocity.

I'm sure George will tidy this up somewhat.

Olly

[Stu]

So we are talking about the 3d equivalent of the surface of a sphere? Tricky to get your head round!

I assume that locally galaxies can have motion which is independent from the expansion of space eg Andromeda approaching the Milky Way?

[FraserClarke]

I assume that locally galaxies can have motion which is independent from the expansion of space eg Andromeda approaching the Milky Way?

Yep, exactly right. You see these 'peculiar velocities' all over the place; where gravity locally overcomes the expansion. Indeed, if you can measure them, you can use them to measure the matter (baryonic and dark) distribution in the Universe.

[ollypenrice]

So we are talking about the 3d equivalent of the surface of a sphere? Tricky to get your head round!

Yes, or not so much tricky as impossible! Join the fun...

I assume that locally galaxies can have motion which is independent from the explsion of space eg Andromeda approaching the Milky Way?

Yes. It is possible to measure the extent to which vast numbers of galaxies are 'beating against the flow' of the general expansion (known as the Hubble Flow.) What emerges is a pattern in which, in certain areas of space, the galaxies are all beating against the flow in a certain direction. Something, therefore, is pulling them that way, a great gravitational force with no obvious visual equivalent, hence ...brrrr... dark matter!!! Eeek.

Great, innit! One of these gravitating areas has the wonderful name of 'The Great Attractor.' I thought that was Carolyn Porco but that's another story!!

Olly

[Stu]

Thanks all for the answers. Olly, now you've mentioned Dark matter, how does that relate to dark energy? I assumed dark matter drove higher than expected gravitational forces and doesn't dark energy drive things apart? Yet another gap in my already sparse knowledge!

[ollypenrice]

I'm getting out of my depth here. Dark energy has a number of book-balancing roles in cosmology. Einstein originally proposed a 'cosmological constant' as an anti-gravity force to allow the universe to find a non-expanding equilibrium because the astronomers told him they had no reason to believe the universe was expanding or contracting. Then Hubble, drawing heavily on Slipher and Shapely without lashing out the credit in their direction, found it was expanding. (At least that's how everyone else interpreted his findings. He was never sure, strangely enough.) So out went the Cosmological Constant. The oft quoted Einstein statement that it was his 'greatest aesthetic blunder' should be treated with caution since the source was the hard drinking, hard-jesting George Gamov!

But now it has come back. I think it can have a number of roles but here we need a physicist.

Olly

[Dandobi]

Come on Dr Brian Cox! Don't you use this forum?!

[Stu]

Where's George when you need him? We're still awaiting the second installment on the different eras of expansion so I'm sure Dark energy would be simple to explain at the same time!

[Holliday]

I'm also pretty much a newbie with the science behind it all, but I like reading my alternative theories and things they can't explain.

Quasars being one I read about recently. I'm sure someone will be able to explain a lot better, but I think the general gist of it was that they don't conform to red shift as they should, scientists claim that these aren't actually within the corresponding galaxies but instead much further behind, but there's evidence of plasma trails connecting them, thus they are in fact pretty much the same distance as the galaxy but don't have the same red shift. Thus red shift can't accurately determine distances... or something along those lines!

As I say, a rather crude explanation but as much as my simple mind can fathom!

Also the Electric Universe theory seems pretty interesting, not read up much on it but I think any theory which legitimately questions our currently accepted theory is worth looking at.

The Whole Big Bang theory, personally, I just can't quite see how everything we see around us, every smell, every sound, taste, feeling, originated from something as big as a raisin!

Isn't Space awesome