I have a question about the age of the universe?

Question

I saw that one of the ways scientists calculate the Universe's age is by seeing how far away they can see, then taking that distance in light years as the Universe's age. I saw that the farthest they can see is about 15 billion light years. My question is, doesn't that prove the universe is actually at least 30 billion years old.

If the universe is expanding, then it would take 15 billion years for the universe to get 15 billion light years big (assuming expantion at the speed of light) and then another period of time for objects to form out there, then another 15 billion light years for the light to get back?

I usually assume if smart people with more knowledge on a subject say something that to me seems obviously wrong, it is that I missed something subtle they know. So what am I missing? Could someone please help?

Answer 1

Your thinking is sound, but to some extent you've got to put aside what seems intuitively correct when talking about the universe in terms of size, age, etc.,.

First, when astronomers say that the universe is this or that age, they're leaving out one very important word -- OBSERVABLE. The observable universe is about 13.7-billion years old. There are unimaginable distances beyond 13.7-billion light years that we will never be able to observe. Two reasons: (1) the more distant some major structure is from us the greater will be its recessional velocity to the tune of 70 km/sec for every 3.26-billion light years. It follows that there are galaxies rushing away from us faster than the speed of light (..this doesn't violate relativity because it's actually space that's expanding..) Light from those galaxies can never get back to us! (2) immediately after the Big Bang, the universe expanded faster than the speed of light for a brief period. Light from objects that today fill that "inflated" space can never reach us either.

One other point that you might want to consider is that the universe has no center. No matter where you are in it, that point seems to be the center because all major structures in all directions are rushing away. If we lived in one of those galaxies 13.7-billion light years away and looked in all directions we'd see the same thing--everything rushing away from us. Consequently it doesn't make sense to try and talk about how big the universe is (..that's supposing that our cosmos isn't inside some "thing" even larger..)

Lots of stuff tossed at you I know, but some of it may give you new insights. I'm at [e-mail] if you'd care to kick this stuff around some more.

Answer 2

Over 15 billion years ago, the objects were not "out there" at a distance of 15 billion light years. They were pretty close together, since that was shortly after the Big Bang. The light from them took a lot less than 15 billion years to reach us, because they were not that far away.

It's difficult to explain this without including some discussion of the expansion of space itself - the fact that space is expanding explains how something that appears the way it did 15 billion years ago can also be at a distance of 15 billion years.

You're better off trying to glean an answer from this article:

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

It's not easy, but it's really the best way the answer this question in as short a space as possible.

Good luck,
Big Al Mintaka

Answer 3

All of the answers assume that the universe is expanding, but there is only observable data that certain celestial objects are becoming more faint visibly or are showing a doplar shift away from Earth; we do not know if they are moving away or if they are being acted upon by another gavity source. All of this makes even less sense if you take into account String Theory. So there is simply no way to tell how old the universe is. All a scientist can tell you is the age of the furthest visible object (via ray detection) based on data collected relatively recently.

Answer 4

Braxton_paul above made a small booboo with the Hubble constant: it is 70 km/s per megapasec, but he put it at "per gigaparsec", which is off 1000 times.

If one multiplies the Hubble constant by the current estimated size of the observable universe (13.7 billion light years, or 4200 megaparsec) we essentially obtain the speed of light. This, in turn, implies that the farther objects are receeding at a speed that is, relative to our position as observers, so close to the speed of light that they are, according to the theory of relativity, "frozen in time". We then see them as they were 13.7 billion years ago, they are moving as fast along with their own image, so to speak. So, the image we have of them is as it was, not as it is or should be now. These objects should indeed have moved further in the interim (while their light was making it back to us), but this is precisely the point: what we see now are images of them as they were 13.7 billion years ago.
The problem is that we cannot assume those objects are on the boundary of space, as this would in some weird way put them is a preferred position, and relativity is supposed to prevent that. They are at the edge of the universe we can observe, but an observer out there sould be able to see something that would be like what we are seeing.
These objects are moving relative to us, but to an observer there, *we* are moving relative to them.
As to the age of the universe, the flaw in the logic is that all those things happened at the same time. The Big Bang occured, the first objects formed in a few million years, and the objects that are further from us pulled away at nearly light speed along with their image from that time. The image we see now is the image of the light that left them at that point, 13.7 billion years ago, and it had to spend all that time just getting to us while the object was moving away. But that image is essentially of the object as it was when it was very close to us. Intuitively, you can even think that the image is dragged along with these objects (although not technically true, as the image does travel at the speed of light, so we do have another image, but the refresh rate is essentially zero, because of relativistic time dilatation).
It is not easy to wrestle oneself from the more comfortable and intuitive Newtonian physics and move on to Einstein's relativity. Give yourself some time to assimilate it; after all it took years to the greatest geniuses of this planet to come up with; no one can really grasp it overnight.

Answer 5

then it should be 45 billion years. because scientist can not measure the distanace faster than speed of light. and for observation they also need to spend that much time.
I think they use some other method to measure the age of universe.