Hubble Deeper Field

[wollery]

There are several posts in this thread that I think I need to respond to , but I'll start here.

But as for the universe going on forever...
It does, at least, go on beyond what's visible. IE: there are, surely, plenty of galaxies more than 13 billion light years from here. It will be a very long time before anyone in this part of space gets to see them. In fact, the universe may be so big that the expansion will move apart some points more quickly than the speed of light.

No two objects in the Universe, no matter how far apart they are, can move relative to each other at a velocity greater than the speed of light. Einstein's theory of special relativity excludes this possibility.

[edited to add] In fact I'm not sure if we would ever see galaxies that are not currently in the visible universe. If the universe became transparent 15 billion years ago (say), and there is a galaxy now 16 billion LY away, then how would we ever see it? [/edit]

The size of the visible Universe expands as the Universe itself gets older. If we were to measure the distances to the furthest galaxies as they are at this time then many of them would be more than 13 billion light years away. However, they are not beyond the edge of the visible Universe. You have to remember that the edge of the visible Universe is less than a billion years old. The light being emitted by a 13 billion year old galaxy that's 16 billion light years away won't reach us for another 25 billion years or so! By which time the Sun and the Earth will be long gone.

 

[CurtC]

Eos wrote:
I heard some galaxies are actually moving closer to ours, but some further away from ours. I just don't understand how this can happen if we are infinitely expanding 'outwards'.

If a friend is driving over to your house, he is getting closer to you. This doesn't violate the idea that, on a large scale, things in the universe are getting farther apart. There are also stars in our galaxy that are getting closer to us. And there are nearby galaxies that are getting closer to us, because we're gravitationally attracted to each other, or both to the same thing. The expansion of space applies at even larger scales. When you get far enough away that local gravitational attractions don't override it, things will be moving away from us, and the farther, the faster.

In my infinite ignorance, I have no idea what cosmic background radiation is.

Just to get you started, right after the Big Bang, the universe was too hot to allow light to propagate. After it was 300000 years old, it had cooled enough to allow photons to pass through. Some of these photons have been travelling that whole 13.7 billion years, and are just now arriving to where we are. And what's more amazing - they're coming pretty much equally from all directions. This faint glow is called the cosmic background radiation, sometimes the cosmic microwave background, since the energy of the photons has been shifted down by the expansion of space into the microwave region (as opposed to the visible light region) of the light spectrum.

 

[wollery]

I heard some galaxies are actually moving closer to ours, but some further away from ours. I just don't understand how this can happen if we are infinitely expanding 'outwards'.

The vast majority of galaxies are moving away from us, but the galaxies in the Local Group (such as M31, the Andromeda Galaxy) are gravitationally bound to each other. This means that some of the Local Group galaxies are moving (sort of) away from us and some are moving (sort of) towards us. What's actually happening is that all of the Local Group galaxies are orbiting the centre of mass of the group. However, the distances and velocities involved mean that we can't actually measure the tangential velocities of the other galaxies, only their radial velocities.

In my infinite ignorance, I have no idea what cosmic background radiation is. I have something to look up now, and hopefully that will explain a few things, thank you. Can you expand on that topic?

Put simply, the cosmic microwave background (CMB) is the afterglow of the big bang. It is a constant, almost invariant "glow" equivalent to a temperature of ~3 Kelvin.

If there was one beginning, then why can't there be several?

There can, some theories have a repetitive Big Bang, Big Crunch, others have multiple universes. We don't know, and probably never will.

Another bit of my ignorance that I hope someone can explain in layman's terms is how we know for sure that "actual space" is getting bigger, and explain what exactly is this "space" that can get bigger if space is essentially nothing. Is the universe getting bigger, or all of space?

First off the Universe is all of space!
The first clue to the expansion of the Universe came from Einsteins' relativity, the simplest solutions of which require the Universe to be either expanding or contracting. Hubble then discovered that almost all of the galaxies were moving away from us, and that the speed of their motion was proportional to their distance away from us, confirming relativities prediction. The clincher though was the CMB, this is also a prediction of relativity, and (so far as we know) can only come from a Big Bang.

Then one of these I may start knowing what the heck I am talking about

For instance, I have no idea what this means:

quote:

By the early 1970's it became clear that the CMB sky is hotter in one direction and cooler in the opposite direction, with the temperature difference being a few mK (or about 0.1% of the overall temperature). The pattern of this temperature variation on the sky is known as a "dipole", and is exactly what is expected if we are moving through the background radiation at high speed in the direction of the hot part. The inference is that our entire local group of galaxies is moving in a particular direction at about 600 km/s. In the direction we are moving the wavelengths of the radiation are squashed together (a blue-shift), making the sky appear hotter there, while in the opposite direction the wavelengths are stretched out (redshift), making the sky appear colder there. When this dipole pattern, due to our motion, is removed, the CMB sky appears incredibly isotropic. Further investigations, including more recent ones by the COBE satellite (eg Smoot et. al.), confirmed the virtual isotropy of the CMB to better than one part in ten-thousand.

http://www.astro.ubc.ca/people/scott/cmb_intro.html

I am extremely curious about all this. I would love to understand it all more.

What part(s) of that don't you understand? I'm happy to help if you can be a bit more specific.

 

[FireGarden]

wollery
This page http://curious.astro.cornell.edu/question.php?number=575 backs up the idea that objects can seperate at the speed of light. IE: the distance between them increases more quickly than c. That increase is due to expansion of space not actual movement.

If we were to measure the distances to the furthest galaxies as they are at this time then many of them would be more than 13 billion light years away. However, they are not beyond the edge of the visible Universe.

Yes, to the first sentence.
I had greatly underestimated what expansion could achieve in the time since the visible galaxies emitted the light we are seeing.

But "No" to the second.
I've already given my explanation, based on the above link, in the "observable universe" thread. As we watch distant galaxies they will appear to slow down and stop (become infinitely redshifted). In fact, they will have continued to have had active lives, but that existence will take place outside the visible universe, which expands - but not quickly enough to retain every object that's currently visible. That's due to recesion at speeds greater than the speed of light.

When we see today's CBR, we see light emitted from a point X that was quite close to us about 13.7 billion years ago. But expansion of space has put that point so far away from us that we cannot see anything that happened at that location after today's CBR was emitted. Tomorrow's CBR comes from a point that, 13.7 billion years ago, was just a little bit further away than X. The visible universe expands only in the sense of seeing further into space as it existed 13.7 billion years ago.

At least, that's what I understand from the above link. (I've had to retract a few things that I posted on this subject since reading that link)

Eos of the Eons
CurtC has explained CBR for us, and he and wollery pointed out that the local group of galaxies (like Andromeda) are gravitionally bound.

The common analogy for expansion is to think of a balloon as it is inflated. If there are two marks painted on the balloon, then those marks cannot move through the surface of the balloon - their positions are fixed. But they can move apart from each other, and for a given rate of inflation, you can get the marks to seperate at any speed if you can get them far enough apart. Even at speeds greater than that of light.

When Einstein first formutlated his General theory of relativity, he came up with a formula that implied the expansion of space. But he thought that that was absurd, so he added a term to cancel it out. He called that his biggest blunder. He could have predicted the expansion.

I don't understand the derivation of that formula, so I'll stop here! (Until I'm better informed, anyway! Ask me in a couple of years! )

If there was one beginning, then why can't there be several?

There may have been more than one, but we are observing the consequences of only one.

The point of my previous post was to give you a couple of things (expansion and CBR) that would need to be explained by any cosmological theory. After all, these are observed phenomena.

 

[FireGarden]

I thought I would highlight this since espritch has already addressed it in the other thread.

and explain what exactly is this "space" that can get bigger if space is essentially nothing.

espritch's reply:
Space is not "nothing". According to relativity, it can be curved by gravity and it can expand (spreading out the matter of the universe in the process). According to quantum mechanics, it is full of zero point energy that is constantly producing and destroying sub atomic virtual particles. Whatever space is, it is certainly not nothing.



I would add that spacetime is the place where events happen.
In the past, spacetime was so small that events could only be seperated by fractions of a second or by fractions of a metre. It's grown since then!