Thread: Expansion of the Universe.

I would be interested in an explanation of the expansion of the universe.
DrRocket has stated that SpeedFreek has a good understanding of this process and I feel that SpeedFreek might be able to give a decent account without becoming too technical.
I used to think I had a reasonable grasp of what was going on but I am much less certain these days.
Anyway, I am always willing to listen to the thoughts of others on this topic.

Originally Posted by Halliday

I would be interested in an explanation of the expansion of the universe.
DrRocket has stated that SpeedFreek has a good understanding of this process and I feel that SpeedFreek might be able to give a decent account without becoming too technical.
I used to think I had a reasonable grasp of what was going on but I am much less certain these days.
Anyway, I am always willing to listen to the thoughts of others on this topic.

While you are waiting for Speedfreek you might want to take a look at these

http://en.wikipedia.org/wiki/Metric_expansion_of_space

http://hepwww.rl.ac.uk/PPDseminars/t..._confusion.pdf

http://arxiv.org/abs/astro-ph/0310808

Well I certainly asked for it, didn't I Doc? :-D

Halliday, seeing as how you have given me little information as to what you know already, you will have to stop me if I start getting too technical. I will start with a highly simplified view and we can go from there. Hopefully you will have taken a look at the links that DrRocket provided above, and if we are lucky they will cover all your questions.

Okay, well the whole point of this is to help tie in what we observe when we look out into the universe with our current cosmological model, the Lambda-Cold Dark Matter Concordance theory. This model describes a universe that is expanding, with a rate of expansion that has been changing during the history of the universe.

At first the rate of expansion was huge, but it quickly decelerated and continued to do so for billions of years. Then, when the universe was something a little over half its current age, the change in the rate of expansion started to turn from a slow deceleration into a slow acceleration and the expansion has continued to accelerate since.

But, what exactly is expanding? Well the answer is known as the "background metric" and it describes the way that cosmological distances have increased, over time. A popular illustration of this is to draw a grid on a sheet of rubber and then stretch the sheet - the grid expands with it. If you are worried what happens at the edges of the sheet, then use a balloon instead! :wink:

The upshot of all this is that as the age of the universe increases, so does the distance between objects that aren't bound together by other forces, such as gravity. In case you are wondering, early on in the history of the universe that rate of expansion was so fast that even gravity (which is the weakest of the forces) wasn't able to do anything but temporarily slow it down.

Interestingly, when we run the expansion backwards the theory predicts there is a point, if we consider back far enough, where the universe first becomes transparent. This is known as the recombination era and it represents a time after which the universe was full of photons moving in random directions that could move freely for the first time without hitting anything!

We assume, as the universe was full of those photons, that they have been arriving here throughout the history of the universe, and it is those photons that we detect today, as Cosmic Microwave Background radiation. They have been effectively stretched into microwaves by the expansion of the universe, or to put it another way, they are being detected in a different frame of reference to that in which they were emitted, and the difference in those frames is due to the change in the background metric!

These CMB photons are the light that has been travelling for the longest time to reach us, as they were released in the earliest event in the universe that it is so far possible for us to detect. They have been tracing a path towards us for nearly 13.7 billion years.

But how far have those photons travelled? Well, that all depends on how you look at it!

There are three main distance measures used in cosmology:
1) Light travel time, or look-back time. This is simply the time it takes light to travel. As the background metric has been changing throughout the time that light has been travelling, except for relatively close cosmological distances light travel time does not accurately reflect a distance through space.

2) Angular diameter distance, or proper distance. This is the distance through space from here to the event, at the time of the event. This can be thought of as where everything was in the past, when the light we see was originally emitted. It is what we actually see.

3) Co-moving radial distance. This is the distance through space from here to the place where that event occurred, if that place is receding with the expansion of the universe. It can be thought of as where everything in the universe is "today", throughout the observable universe.

When it comes to those CMB photons, they have a light travel time of nearly 13.7 billion years, the place they were released from had a proper distance of only a little over 40 million light-years when they were released (yes, it is that small!), and that place "today" has a co-moving radial distance of over 46 billion light-years.

The galaxy whose light has been travelling the longest to reach us has a light travel time of 13 billion years, was originally 3 billion light-years away when it emitted the light we see, and is now around 30 billion light-years away as we detect that light.

But the matter that makes up that galaxy was originally closer to us than 40 million light-years away at the time the CMBR was released. Everything in the observable universe was originally closer to us than 40 million light-years away back then. The CMBR represents a cosmological horizon known as the particle horizon and it is the edge of the observable universe, the light that was emitted from the place now most distant from us.

Any questions so far? Anything specific you want me to try to cover?

Okay, now for some visuals, to illustrate the above.

This is a space-time diagram.


Image credit - Prof. Mark Whittle, http://www.astro.virginia.edu/~dmw8f/ - from the extragalactic astronomy section

So, what are we looking at here?

Well, let's start at the top left, where it says Here & Now. The vertical axis that "Here & Now" sits on marks light travel time, or lookback time. As you trace that axis downwards you are looking backwards in time, and you reach the origin at the Big Bang something over 13 billion years ago.

This vertical axis itself represents our worldline - it represents "here" across the history of the universe, at rest in relation to the expansion of the universe.

The horizontal axes, both higher and lower, represent proper distance - that is distance as measured by a ruler anchored "here", and the ruler does not expand with the universe.

What expands is the proper distance to other galaxies, which are shown as blue worldlines. Notice that all worldlines converge at the Big Bang.

So, how does this all relate to what we see?

Well, the red line shows what we see, here & now, which is known as our past-light cone. The light of all the galaxies we can see right now traced a path along that line towards us, from the place where the worldines of those galaxies intersected that red line. The whole of the red line represents the theoretical path of a photon released at recombination, relatively close to "here".

Let's start with the middle galaxy, one we measure here & now as having a redshift of z=1. See where it crosses our lightcone, marked - this represents the distance at the time the light was emitted. If you trace a vertical line downwards from there, it intersects the horizontal axis at 5.2 billion light-years. That was the proper distance to that galaxy (), at the time the light was emitted. If you instead trace from the intersection point horizontally towards the vertical axis, it shows that the light took 7.3 billion years to reach us (shown as ).

Now look along the upper horizontal axis and see where that galaxy's blue worldline intersects it at 10 billion light years - that is the distance to the galaxy "now", which is known as the co-moving radial distance, or the distance at the time the light is observed ().

So, when we are looking at a galaxy with a redshift of z=1, we are seeing a galaxy as it was 7.3 billion years ago, when it was 5.2 billion light-years away. Today, it would be something around 10 billion light-years away, due to the continued expansion of the universe since the light we see was originally emitted.

Now look at a more distant galaxy, the right hand blue worldline with a redshift of z=6.8. It crosses our light cone 12.6 billion years ago, when it was only 3.6 billion light-years away, but today it is 28 billion light-years away (need a wider diagram? check out the top diagrams on pages 3 and 11 of the pdf file at the last link in DrRockets post).

That galaxy has always been more distant than the galaxy we see at z=1, but we see them at different times in the history of the universe. Our light cone cuts a slice through the universe all the way back to the time of recombination, when everything, including all the stuff that made up those galaxies, was very close to "here" indeed.

See what else you can get from the diagram, and the ones in the pdf at DrRocket's last link. Then, check out the links below!

http://www.astro.virginia.edu/class/whittle/astr553/
http://www.astro.virginia.edu/class/...ght_cones.html

Pure awesome. I was just a passive observer, but want to post a thank you for that. It's accessible, so cheers.

The infinate universe does not expand. Infinate doesn't get any bigger. Start, change, beginning and end are human constructs. In the infinate universe all you can guarantee is change. In the infinate universe studying, learning and more to do tomorrow are always guaranteed. The "earth is flat" " the universe is finite" same thing.

Originally Posted by Harold1948

The infinate (sic) universe does not expand.

Then how do you explain galactic redshifts and the CMBR?

Originally Posted by Harold1948

Infinate (sic) doesn't get any bigger.

Are there more integers than there are positive integers alone?

Originally Posted by Harold1948

Start, change, beginning and end are human constructs. .

Was there no change before humans evolved? How did theydo so without change?

Great job SpeedFreek!

http://www.c-spanvideo.org/program/HiddenR

Thanks for all that info SpeedFreek, it certainly helped

Originally Posted by Harold1948

The infinate universe does not expand. Infinate doesn't get any bigger. Start, change, beginning and end are human constructs. In the infinate universe all you can guarantee is change. In the infinate universe studying, learning and more to do tomorrow are always guaranteed. The "earth is flat" " the universe is finite" same thing.

rubbish

Originally Posted by Harold1948

The infinate universe does not expand. Infinate doesn't get any bigger. Start, change, beginning and end are human constructs. In the infinate universe all you can guarantee is change. In the infinate universe studying, learning and more to do tomorrow are always guaranteed. The "earth is flat" " the universe is finite" same thing.

Tell you what, read all of the articles that DrRocket put up. Also, go to all of the pages of the words that were highlighted in the Wikipedia page to get the overall point summed up. The things that you are talking about are your own "theories", I guess... So just get familiarized with the reality of the topic.

here is also mentioned expanding of universe... which may be an optical illusion!
http://universemyperception.blogspot.com/

Originally Posted by sandi

here is also mentioned expanding of universe... which may be an optical illusion!
http://universemyperception.blogspot.com/

Freaking pile of crap BS. No one go there. Uggh!

Really good explanation SpeedFreek.
I was just wondering, is this forum full of under and post graduates cause I feel like the youngest one here and obviously in the receiving position.

Originally Posted by Nilakandan

I was just wondering, is this forum full of under and post graduates cause I feel like the youngest one here and obviously in the receiving position.

People of all ages, backgrounds, experience, and education levels. Some high school students. Some people who don't do science, but are interested in it. Some people actively publishing or working in fields. Some people who are morons spreading their tripe across the net like flaming pile of stink. We've got them all.