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Infinitenight2093
2010-Nov-06, 07:09 PM
Is the diameter of the universe a measure of how large the universe should be today? (ex. the furthest galaxy that we can observe in the universe would now be 46 billion LY away?)

George
2010-Nov-06, 08:13 PM
Is the diameter of the universe a measure of how large the universe should be today? (ex. the furthest galaxy that we can observe in the universe would now be 46 billion LY away?) If you could stop time everywhere in the universe and yet be able to go out and measure it to the furthest point we can "now" see, it would be, I think, about 46 billion light years radius based on the current mainstream expansion model for the universe. This would be the distance to the light we now see as the CMBR, which is the light that came from the amazing flash of recombination -- when things cooled to allow electrons to be contained by protons causing light to be free from the annoyance of electrons everywhere. This light was, at the time of recombination, only about 41 million light years from our position, though I'm going on information I have learned here in BAUT without researching it.

speedfreek
2010-Nov-07, 11:33 AM
Yes, as George said, the observable universe has a radius of around 46 billion light years and that radius is defined by the source of the light that has travelled the longest to reach us. The "earliest" light we see comes from the Cosmic Microwave Background Radiation, which was released throughout the universe around 400,000 years after the Big-Bang, in an event called recombination.

When the CMBR photons we detect today (coming in from all directions) were originally released, the universe was around 1100 times smaller than it is today. The universe was expanding really fast, and it took 13.7 billion years for CMBR photons released only a little over 40 million light-years away to reach us, during which time the universe expanded to 1100 times the size, meaning that the release coordinates for those photons will now be 1100 times more distant - 46 billion light-years away, in all directions. It is a conceptual sphere around us that marks the extent of the universe we have received light from.

As for galaxies, the most distant galaxy we have seen is an early galaxy that existed only 600 million years after the Big-Bang. The galaxy was only 3.1 billion light-years away at that time and the light from that galaxy has been travelling towards us through the expanding universe for 13 billion years. That galaxy (or whatever became of it!) is now around 30 billion light-years away.

Ken G
2010-Nov-07, 02:49 PM
Reasoning like that shows the awesome power of the cosmological principle!

Infinitenight2093
2010-Nov-07, 05:55 PM
so would there ever be any hope of us peering 46 billion LY into the universe or are we stuck at the point of recombination?
Also, does the hubble volume have anything to do with the age of the universe or is it solely based on the expansion rate of the universe?

CosmicUnderstanding
2010-Nov-07, 06:24 PM
I can't even wrap my head around a few thousand light years, let alone billions of light years. My gosh...the distance..

Ken G
2010-Nov-07, 06:26 PM
so would there ever be any hope of us peering 46 billion LY into the universe or are we stuck at the point of recombination?Not with light, it can't get through. But we can use light from 46 billion LY away to indirectly infer about the processes that gave rise to that light, from even greater distances.

Also, does the hubble volume have anything to do with the age of the universe or is it solely based on the expansion rate of the universe?It's purely about the current expansion rate, nothing about age, and that's part of why it is a fairly useless concept. The 46 billion LY "radius of the observable universe" is a much more important distance.

kevin1981
2010-Nov-08, 10:10 PM
Is the reason we can't see beyound the "observable universe" because of "dark energy" increasing the expansion rate and the light beyound that can not reach us, due to the speed of light restriction. How do we know that their is more space beyound the horizon?

Thanks

speedfreek
2010-Nov-09, 12:04 AM
Is the reason we can't see beyound the "observable universe" because of "dark energy" increasing the expansion rate and the light beyound that can not reach us, due to the speed of light restriction.
No, not in the usual sense of the term, but it all depends on how you define the "observable universe". You are actually describing what is known as the cosmological event horizon, which is a different concept in cosmology to the observable universe.

The standard definition of the observable universe, as used in cosmology, is based on the time that light has had in which to travel. If the universe began 13.7 billion years ago, then light can only have had a maximum of 13.7 billion years in which to reach us. The light that takes the longest time to reach us will have travelled the furthest and will have come from events that happened earliest in the history of the universe.

The edge of the observable universe is an edge beyond which there is no more to see, because there was nothing to see back then!

The earliest event we have detected light from is an event called recombination, which happened sometime around 400,000 years after the Big-Bang. It was during this event that the universe became transparent and light was first able to move freely. The universe was filled with light at that time, and it is this light that we detect as the Cosmic Background Radiation (CBR), coming at us from all directions, nearly 13.7 billion years later.

During the time between recombination and today, the universe has expanded tremendously, so the place that the CBR that we currently detect was originally emitted from, if it has receded with that expansion, would today be something around 46 billion light-years away. This is the edge of our observable universe, as it marks the place from which we have detected the light that has been travelling for the longest time. The observable universe today is a sphere with a radius of 46 billion light-years, with us in the centre. We assume that "today" there are galaxies at the edge of our observable universe just as there are galaxies over here, but due to the finite speed of light, all we see is radiation released there long ago, when the universe was smaller and "there" was a lot closer!


How do we know that their is more space beyound the horizon?

We also assume this region of the universe we live in has been constantly bombarded by the CBR since recombination - we are constantly being hit by CBR originally released at greater and greater distance! We assume that we will continue to detect the CBR for a long time to come. This means we are detecting light that was originally released at a greater distance away than the CBR we currently detect, which means there is more space in the universe as a whole than in our current "observable universe".

But, there is something else that puts limits on our view of the universe - dark energy. This limits what is observable in the future, due to the acceleration of the expansion of the universe, and causes what is known as the cosmological event horizon.

Here, the accelerating rate of expansion causes an apparent recession speed at a certain distance (in all directions!) that limits the events we will be able to see in the future, due to the light from those events not being able to reach us, as you said.

And of course, we assume that an observer anywhere in the observable universe would see pretty much the same thing we do, so all the above applies everywhere!

Colby
2011-Feb-05, 12:33 AM
When the CMBR photons we detect today (coming in from all directions) were originally released, the universe was around 1100 times smaller than it is today.
So, the universe was 13.7 / 1100 = 12M LYs radius at the time of the CMB? The inflationary epoch ended well within the first second and was only 10cm across. How did the universe get to be 12M LYs radius in only 400k years? The expansion was faster than light speed, I assume, but I'd like confirmation, correction, or clarification. Thanks.

speedfreek
2011-Feb-05, 01:46 AM
In an expanding universe the 13.7 billion figure relates to time, rather than distance. You need to divide the current distance to the edge of the observable universe as described earlier in the thread, 46 billion light-years, by 1100, to find the radius of the observable universe at the time the CMB was released and the answer is a radius of around 41 million light-years. The CMB we detect today was originally released only ~41 million light-years away, only ~400,000 years after the Big Bang.

So yes, the apparent recession speed of the "particle horizon", the edge of the observable universe, was faster than light speed at that time. Even though the rate of expansion had been decelerating for 400,000 years, the particle horizon was receding at over 50 times the speed of light at that time. Today, the edge of the observable universe is still receding at over 3 times the speed of light.

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

WayneFrancis
2011-Feb-05, 01:44 PM
Again I say with an expanding space model if the expansion is the same every where then you'll find 2 points that are expanding faster then c as long as the universe isn't so small that it wraps back on itself to soon. So pretty much for the entire 13.7 billion years the universe has been in existence there have been points that have been receding from each other at a rate > c

melech
2011-Feb-06, 07:18 PM
Still not quite clear on precise definitions. Does "observable universe" refer to the 13.7 billion years the light has been traveling, or the 46 billion years of the present radius of the universe?

speedfreek
2011-Feb-06, 11:41 PM
Still not quite clear on precise definitions. Does "observable universe" refer to the 13.7 billion years the light has been traveling, or the 46 billion years of the present radius of the universe?

Both. One is a measure of the time that the light (or in this case, the CMB radiation) has been travelling for, and the other is the distance to the place that radiation was released from, if that distance has increased with the expansion of the universe. If the CMB radiation is coming in at us from all directions, then we can think of the place where that radiation was released as a sphere around us, currently 46 billion light-years in radius, due to the expansion of the universe.