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Old 01-27-2008, 08:15 PM   #1
popeye18
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Rate of expansion of the universe

This might be a really idiotic question but im quite curious about something. According to the big bang theory the universe began as a singularity and rapidly expanded. Estimates of the age of the universe are around 13 billion years, and the size 95 billion light years.

Physics dictates that nothing can travel faster than light, but since the universe is 95 billion light years across and only 15 billion years old, wouldnt matter have had to travel faster than light to travel so far? Im also under the impression that while still expanding now much of the expansion took place very early in the life of the universe, making the speed even greater. Am i overlooking some fundamental aspect or something?
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Old 01-27-2008, 08:29 PM   #2
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Re: Rate of expansion of the universe

Space itself stretches, so it's not a problem.
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Old 01-27-2008, 08:52 PM   #3
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Re: Rate of expansion of the universe

what do you mean the "size"? if the size is the total area of the universe then there is no problem, since that 13 billion years is essentially a maximum radius
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Old 01-27-2008, 10:24 PM   #4
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Re: Rate of expansion of the universe

Inflation caused the universe in its early quantum state to expand much faster than the speed of light.

The speed of light is a limit for the transmission of information.

Space (apparently!) can expand faster than the speed of light.
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Old 01-28-2008, 09:23 AM   #5
Insp. Clue!So?
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Re: Rate of expansion of the universe

Quote:
Originally Posted by furyshade View Post
what do you mean the "size"? if the size is the total area of the universe then there is no problem, since that 13 billion years is essentially a maximum radius
No, that's just the size of the light cone. Some estimates put the actual size of the universe at 10^10^14 (miles, parsecs, feet, units don't matter at that scale). All we see is a teeny tiny little local patch, what was around us at the time of the Big Bang.
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Old 01-30-2008, 12:02 AM   #6
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Re: Rate of expansion of the universe

Paraphrased; "Space stretches, and that stretching of space totally changes the perception of time. Imagine in your mind going back billions of years ago to the beginning of time. Now pretend way back at the beginning of time, when time grabs hold, there's an intelligent community. (It's totally fictitious.) Imagine that the intelligent community has a laser, and it's going to shoot out a blast of light, and every second it's going to pulse. Every second -- pulse. Pulse. Pulse. It shoots the light out, and then billions of years later, way far down the time line, we here on Earth have a big satellite dish, and we receive that pulse of light. And on that pulse of light is imprinted (printing information on light is called fiber optics - sending information by light), "I'm sending you a pulse every second." And then a second goes by and the next pulse is sent.

Now light travels 300 million meters per second. So the two light pulses are separated by 300 million meters at the beginning. Now they travel through space for billions of years, and they're going to reach the Earth billions of years later. But wait a minute. Is the universe static? No. The universe is expanding. That's the cosmology of the universe. And that mean it's expanding into an empty space outside the universe. There's only the universe. There is no space outside the universe. The universe expands by space stretching. So as these pulses go through billions of years of travelling, and the universe is stretching, and space is stretching, what's happening to these pulses? The space between them is also stretching. So the pulses really get further and further apart. Billions of years later, when the first pulse arrives, we say, "Wow - a pulse!" And written on it is "I'm sending you a pulse every second." You call all your friends, and you wait for the next pulse to arrive. Does it arrive another second later? No! A year later? Maybe not. Maybe billions of years later. Because depending on how much time this pulse of light has traveled through space, will determine the amount of stretching that has occurred. That's standard cosmology.

Today, we look at time going backward. We see 15 billion years. Looking forward from when the universe is very small - billions of times smaller. What's exciting about the last decade in cosmology is we now have quantified the data to know the relationship of the "view of time" from the beginning, relative to the "view of time" today. It's not science fiction any longer. Any one of a dozen physics text books all bring the same number. The general relationship between time near the beginning and time today is a million million. That's a 1 with 12 zeros after it. So when a view from the beginning looking forward says "I'm sending you a pulse every second," would we see it every second? No. We'd see it every million million seconds. Because that's the stretching effect of the expansion of the universe.

Every time the universe doubles, the perception of time is cut in half. Now when the universe was small, it was doubling very rapidly. But as the universe gets bigger, the doubling time gets exponentially longer. This rate of expansion is quoted in "The Principles of Physical Cosmology," a textbook that is used literally around the world.

(In case you want to know, this exponential rate of expansion has a specific number averaged at 10 to the 12th power. That is in fact the temperature of quark confinement, when matter freezes out of the energy: 10.9 times 10 to the 12th power Kelvin degrees divided by (or the ratio to) the temperature of the universe today, 2.73 degrees. That's the initial ratio which changes exponentially as the universe expands.)" -Gerald Schroeder
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