Q&A With Dr. Harry Ferguson
Astronomer, Space Telescope Science Institute

Duncan T Emmons
Hermoston, Oregon

Does our universe go on forever, or is there an end?

Dear Duncan,
the destiny of our universe is regulated by the amount of matter and energy, therefore the amount of self-gravity, that it contains.

The higher the content of mass and energy, the larger the self-gravity. Exactly like a more massive planets, e.g. Jupiter, has more gravity than less massive planets, like the Earth.

If the self-gravity of the universe is enough to stop the expansion, the universe will eventually come to a rest and recollapse (Big Crunch).

If the self-gravity is not enough to stop the cosmic expansion (or if it needs an infinite amount of time to do it) the universe will expand forever.

Present measure on the amount of mass and energy of the universe are highly uncertain. Some data seem to suggest that there in not enough self-gravity. Other data might dispute these findings. We need to keep studying this matter.

Take care,
M. G.

Chris Parkinson
Redwood City, California

I am a little confused at something here. I was taught that quasars are at the rim of the universe, traveling there at tremendous speeds. How can we view them if they should be the oldest remnants of our universe when we can also view the oldest remnants of our universe vis a vis the recent HST deep sky research that shows ancient galaxies forming. Or to put it another way, how can we view two totally different sets of objects, that to me come from the same period in time and space and yet exist in vastly different points in space?

Thanks in advance

Dear Chris,
While it is true that quasars can be observed at very remote cosmic distances because they are very bright, not all quasars are so distant. In fact, there is a fairly large number of relatively nearby quasars.

It is a recent news, actually, that the most distant object known is not anymore a quasar, but a galaxy at redshift z=4.9 discovered thanks to the magnifying gravitational lens of a massive cluster of galaxies in the foreground.

We can see objects at different distances. But we see them at a different age, because the speed of light is finite. In other words, the light from a relatively nearby object took only a modest amount of time to travel from it to us, thus we see that object as it was when the light left it (so, only a modest amount of time ago).

The light from a very distant object, on the other hand, took a much longer time to travel through the cosmos and reach us. Thus, the information that it is bringing to us is the one when it left the object, namely many billion years ago, when the object was young.

Hope this will clarify the picture.

Thanks for writing and take care!!
M. G.

Neil Healey
Montgomery, Alabama

Your broadcast last night provided some staggering statistics regarding the immensity of the universe. However, I wondered if there is any way that astronomers can determine where the Milky Way sits within the universe. Does the universe have a center? If its constantly expanding, can I asume that we wouldn't be located toward the outer limits, because it may be too new? Can we use conventional shapes to even describe the universe? Great show. I'm looking forward to watching again next week. Thanks.

Dear Neil,
No, the universe does not have a center from which originates. The expansion started everywhere at the same time. For a 2-dimensional analogous case, imagine a membrane of cellophane that is being stretched from every direction without breaking, indefinitely. Imagine that this membreane is very, very large, actually infinite. How can you tell that there is a center? Every point is receiding from every other point, and no center can be defined.

That should give you an idea in 2-dimension. It requires a little bit more of geometrical imagination to figure out what happens in 3 dimensions, but it is qualitatively similar.

So, we cannot place where the Milky Way is in the universe...

Thanks for writing us.

Take care,
M. G.

Tom Tillander
Cleveland Ohio

Two pieces of information that i absorbed from Mondays program are:
1. The the light that was imaged by the deep space team has been traveling toward us for 14 billion years.

2. The team believes that the universe is 15 billion years old.

One piece of information I didn't catch was the relative speed of separation between us and those galaxies. For the sake of this question I will assume it is .5C

Given that assumption, in the 14 billion years since the light left those deep space galaxies we could have only increased our separation from them by 7 billion light years. That means that at the time the light left those galaxies we were already 7 billion light years away.

If the big bang occurred at a point. How did we separate that initial 7 billion light years in only 1 billion years?

I suppose my real question is where is the flaw in my thinking?

Thanks

Dear Tom,
14 billion years ago the galaxies were all physically closer to each other by an amount that it is, more or less, equal to the rate of expansion (whose exact value is still uncertain and will require more measures to be done) times the time that they have been expanding, namely 12 billion years.

The redshift that we see from distant galaxies is caused by their recession velocity. However, bear in mind that this velocity is not a physical one in the sense we are used to. It is actually entirely due to the expansion of the space and not to a physical motion (well, there are also physical motions, the so called "peculiar velocity", but they contribute only a minor amount to the observed redshifts).

The "cosmological redshifts" are cause by the "dilatation" of the space between two galaxies (a consequence of the cosmological expansion). Thus, the velocity involved are very high, larger than the speed of light if you compute them using the traditional doppler formula. However, this does not violete general relativiy, because the motions, as I said before, are not physical motions but due to the cosmic expansion.

Thus, the flaw in your reasoning was to assume a speed of 0.5c. But actually the galaxies were not moving in a physical sense. It was the space between them that expanded a lot. This is the expansion that causes the redshift, not a proper motion in the space. And this expansion was very very large in the past, much larger than 0.5c. Still, no contraddiction with General Relativity which dictates that only physical motions have to be smaller than c.

Hope this clarifies things a little.

Take care and have fun with the program.
M. G.

Gerald Lame
Palm Springs, California

One of the astronomers on the first program indicated that, although the distance we can see into the universe is limited by the amount of time light has had to reach us since the big bang, the universe in fact extends beyond this limit, which I think he called our "event horizon". But how did anything have enough time to travel that far since the big bang?

If everything started at the same place, at the time of the big bang, and no object can travel relative to another faster than the speed of light, how could any object have time enough to travel away from us more light years than the age of the universe?

Dear Gerald,
Your confusion originates from the fact that you imagine the Big Bang originating from one point in space. But in fact the Big Bang took place everywhere at the same time, with no center in it.

You can have a 2-dimensional equivalent if you imagine a cellophane membrane that is being stretched from every direction in a homogeneous way. Imagine that the membrane will keep extending without breaking. Imagine now that the membrane is very, very, very large. Actually, infinite. Where is the center? There is no center there, right? Every two point keep expanding from each other as far as the membrane is being stretched and there is no need to even think of a center.

Now imagine a point on this membrane that is shining light. In a given time, only points close enough that the light had time to reach will see the beacon. The other points will have to wait a little bit longer, until the light hits them. But they are there, just out of reach of the shining one.

This is a good examply, albeit in 2-dimensions only (it takes a lot of abstraction to figure out things in 3 dimensions in this case), of the universe.

In summary, the expansion of the universe, which is not a physical motion but only a dilatation of the space, was faster than the speed of light, with no violation of General Relativity, which prescribes that only physical motions cannot be fater than the light. The expansion of the space is not contrained by that law. Because the expansion was faster than the light (at the very instant of the Big Bang it was actually infinite) there are points in the universe that light did not have yet the time to place in causal relation (namely, to illuminate). It will do so, though, in due course!

Thanks for asking and take care!
M. G.