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Speed/rate of expansion of the Universe
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hi i was dumbfounded by the revelation that the universe is expanding at an increasing rate, but after much searching still can't find out the numbers involved!
how fast is it expanding (roughly)?
what is the rate of acceleration?
thnx
how fast is it expanding (roughly)?
what is the rate of acceleration?
thnx
Answers
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The term you are searching for is the Hubble Constant and you'll find there is a wide variation and even disagreement on it's ultimate value. We do know, from measurements, that the universe may be expanding faster than the speed of light (c). Some of the galaxies we can see right now are currently moving away from us faster than the speed of light. This is best understood in that some galaxies, relative to each other, move apart at faster than c. The best guess on the Hubble Constant is near 0.007% per million years ... meaning is that every million years, all the distances in the universe stretch by 0.007%. The actual formula involves kilometers per second per megaparsec, and can be confusing... at least to me...
This question has been asked numerous times in this section, and it's certainly a difficult concept to understand. It's entirely oversimplified, but the basic fact is that the expansion of the universe is classically described as an inflating balloon. If you can picture a series of dots on the surface of the balloon you can visualize their "pulling apart" as the balloon inflates. This is what's happening with the known universe. Simply put, there is nothing for it to inflate into since all that is is carried on and within the balloon.
Even more mind boggling is the beginning of all this... "The Big Bang"... except there was no bang and at its inception it certainly wasn't big. In fact there was nothing and suddenly (less than Planck time, but that's another thread) all that was to become our universe inflated from an initial size of 10-26 meters in diameter (a hundred billion times smaller than a proton) to approximately one hundred million light years (10-24 m) in diameter. This occurred in an undescribably (except matmatically) small amount of time. Light from photons appeared and the universe appeared, according to one astrophysicist, to have occurred everywhere at the same time...
Even more mind boggling is the beginning of all this... "The Big Bang"... except there was no bang and at its inception it certainly wasn't big. In fact there was nothing and suddenly (less than Planck time, but that's another thread) all that was to become our universe inflated from an initial size of 10-26 meters in diameter (a hundred billion times smaller than a proton) to approximately one hundred million light years (10-24 m) in diameter. This occurred in an undescribably (except matmatically) small amount of time. Light from photons appeared and the universe appeared, according to one astrophysicist, to have occurred everywhere at the same time...
Geo, the answer should have been explained even further to describe the fact that nothing (so far observed) can exceed the speed of light (c). No galaxy is actually moving faster than the speed of light. However, a quote from Ask and Astronomer might help:
'The bottom line is that different pairs of galaxies are moving at different speeds with respect to each other; the further the galaxies are, the faster they move apart. So when we ask whether the universe is "expanding faster than the speed of light," I am going to interpret that to mean, "Are there any two galaxies in the universe which are moving faster than the speed of light with respect to each other?"
If we use the Hubble Constant for the only measurement of expansion speed in the universe, then it tells us that for every megaparsec of distance between two galaxies, the apparent speed at which the galaxies move apart from each other is greater by 71 kilometers per second. Since we know that the speed of light is around 300,000 kilometers per second (186,000 miles per second), it is easy to calculate how far away two galaxies must be in order to be moving away from each other faster than the speed of light. The answer we get is that the two galaxies must be separated by around 4,200 megaparsecs (130,000,000,000,000,000,000,000 kilometers). Additionally, it's been found found that if you use a value of around 1.4 for z (the redshift), you get the required distance of 4,200 megaparsecs. Therefore, any galaxy with a redshift greater than 1.4 is currently moving away from us faster than the speed of light. Bright galaxies are regularly detected out to redshifts of around 5...
'The bottom line is that different pairs of galaxies are moving at different speeds with respect to each other; the further the galaxies are, the faster they move apart. So when we ask whether the universe is "expanding faster than the speed of light," I am going to interpret that to mean, "Are there any two galaxies in the universe which are moving faster than the speed of light with respect to each other?"
If we use the Hubble Constant for the only measurement of expansion speed in the universe, then it tells us that for every megaparsec of distance between two galaxies, the apparent speed at which the galaxies move apart from each other is greater by 71 kilometers per second. Since we know that the speed of light is around 300,000 kilometers per second (186,000 miles per second), it is easy to calculate how far away two galaxies must be in order to be moving away from each other faster than the speed of light. The answer we get is that the two galaxies must be separated by around 4,200 megaparsecs (130,000,000,000,000,000,000,000 kilometers). Additionally, it's been found found that if you use a value of around 1.4 for z (the redshift), you get the required distance of 4,200 megaparsecs. Therefore, any galaxy with a redshift greater than 1.4 is currently moving away from us faster than the speed of light. Bright galaxies are regularly detected out to redshifts of around 5...
Further, the Inflation Theory states that within 10 to the minus 33 seconds following the "Big Bang" the universe expanded to about 100,000 billion light years in size as previously stated. This certainly couldn't have occurred without the exceeding of c. Right now we only observe 4 dimensions if you count time. It's theorized that at the moment of the Big Bang and throughout the inflationary period there might have been as many as 12 or more dimensions... but those can only be described and theorized mathmatically... certainly beyond my comprehension...
The speed of light is an absolute speed limit for objects moving within our universe.
However when we're talking the universe itself expanding such rules may or may not apply hence the suggestion of the inflationary stage where the universe expanded at a rate greater than that of the speed of light is not necessarily a problem.
Treenor4ish if you were dumbfounded at an accelerating rate of expansion you were in very good company pretty much all of the scientific community were too!
It's of course not the Hubble constant that describes the rate of expansion but the value of the cosmological constant (lambda) that describes the acceleration of the expansion.
It's got a real chequered history - Einstein put it in, then said he wished he hadn't ("my greatest mistake") and now of course it's back with a vengence.
have a read here: http://en.wikipedia.org/wiki/Cosmological_constant
and follow the link for dark energy too.
I can't get over the contradiction here. Einstein and his pals told us that 'nothing (apart from light) can move at or faster than the speed of light. Now we are asked to believe that entire galaxies are tearing away FASTER than the speed of light (relative to us).
Furthermore we can still see them...
I can accept (almost) that they could be receding faster than light, but if we can still see them the light coming back to us would itself have had to travel faster than light.
As surprising as it may seem, jake, I disagree with your statement that "it's not the Hubble Constant that describes the rate of expansion, but the value of the cosmological constant (lambda) that describes the acceleration of the expansion".
The there is a tension between the cosmological constant and matter as to whether or not the universe expands, contracts or is static. Therefore, the rate of expansion (that we observe to exist a la redshift) is controlled, if you will by the cosmological constant, but says nothing about the actual rate of expansion described by Hubble.
Additioanlly, "... while the amount of expansion undergone in any one second by a typical cubic centimeter in such a universe (accelerating) is a constant, the number of centimeters between us and a distant galaxy will be increasing with time; such a galaxy will therefore be seen to have an apparent recession velocity that grows ever larger. (Source: Sean M. Carroll, U. of Chicago)
... in my opinion...
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