Ask Us

Space Physics:
Cosmology - Expansion of the Universe

Expansion of the Universe

I read recently that there is a consensus among many top scientists that the Universe is ever expanding and that there seems to be proof of this now. I further understood that this discovery and agreement among the scientists was the great discovery of 1998, perhaps of the century. Sadly I can't find the article or related articles. Could you possibly refer me to somewhere I could again access this information?

Science Magazine named the evidence (not proof) that the Universe's expansion is accelerating as the "Breakthrough of the Year" for 1998. You can get more information from this article at the Lawrence Berkeley National Lab.

Dr. Eric Christian

Alexander Friedmann and Expansion of the Universe

Whenever I search for information about the Universe, the name Friedmann appears regularly. Who is Friedmann and what did he do?

This is probably a reference to Alexander Friedmann. There are good web pages on his life and work at the University of St. Andrews.

Beth Barbier

Universe Expands Faster Than Light?

Did the Universe expand faster than light? If not, why does light, up to 12 billion years old, reach us only now?

The Universe did not expand faster than light. The Universe was big enough 12 billion years ago that the light from some distant objects is only getting to us now. That doesn't mean that the Universe was more than 12 billion light years wide 12 billion years ago. Because we're moving away from the object, the light has had to catch up to us.

Dr. Eric Christian

Light and the Expansion of the Universe

I saw a story on WMAP that featured images of the Universe when it was only 380,000 years old. If the Universe is 13.7 billion years old, why did it take so long for this light to catch up with us? How did our our galaxy outrun the light that was emitted a mere 380,000 years after the Big Bang?

You are indeed touching upon a very perplexing situation. At an age of 380,000 years (after the Big Bang) the "observable universe", i.e. the entire volume of the Universe that we can see in electromagnetic waves (light, microwaves, radio, etc.), is so incredibly small (with a radius of only 380,000 light years, give or take a few, depending on how much the Universe is accelerating). Yet it took the microwaves of that afterglow originating from its outskirts 13.7 billion years to reach us.

The problem is that the entire Universe has been and still is expanding at an incredible rate. So, what was a miniscule distance of 380,000 light years back then turned into the enormous distance of 13.7 billion light years as of today. The light from the afterglow, on its way, had to cross that ever-expanding space and was literally "stretched" along with that expansion. Its wavelength increased from that of visible light to that of microwaves, which we see today.

To imagine this situation think of the Universe as of a gigantic - maybe even infinitely large - dough of raisin bread. Scale your imagination down to a finite piece of dough, where you are sitting on a raisin in its center. The outer edge of the dough is equivalent to the edge of the "observable universe", from where we see the cosmic microwave background radiation. You have put a lot of yeast into your raisin bread, and you have set up microwave transmitters all around on the outer edge, blasting inward. Imagine that in reaction to this heating the dough rises at such a furious rate that the outer edge recedes from the center at almost the speed of light. Although the microwave transmitters were only at a relatively short distance at the beginning of this "gedankenexperiment" (thought experiment), the microwaves have to cross a much larger (continually growing) distance because of the ongoing expansion.

I would like to add a cautionary note, in case you might be wondering. Don't get confused! This expansion does not violate Einstein's theory of relativity, even though the imaginary dough of the even larger Universe, which we can't see beyond that edge, appears to recede at speeds larger than the speed of light. The dough represents space itself, and in our expanding Universe space itself is expanding, carrying the galaxies (represented by the raisins) along on a ride. Einstein's limit to the speed of light applies only to motion through space, and not to expansion of space itself.

Dr. Eberhard Moebius
(April 2003)


Flat Universe

I just read that the Universe is flat. How does this relate to the thickness of the Universe?

"Flat" refers to the expansion of the Universe, not the size or shape.

Dr. Eric Christian and Beth Barbier

Boomerang Experiment

I read that, according to the data from the Boomerang experiment, the Universe seems to be flat. Does this agree with General Relativity that predicts a bent spacetime? After this discovery, which models of Big Bang will survive?

I think you should go to the source, check out the BOOMERANG home page.

Their results do seem to imply that the Universe is flat, however this in no way contradicts General Relativity, which allows for many different geometries depending on the amount of matter and radiation in the Universe.

Dr. Louis Barbier

How Fast is the Universe Expanding?

Please explain how fast the Universe is expanding.

It appears that the Universe is expanding at 80 km/sec/Mpc (statistical error = 17 km/sec/Mpc), as calculated by the Hubble Space Telescope's Key Project team (Mpc is megaparsec = 3.26 million light years). What this means is that objects will, on the average, be moving away from us at 80 km/sec for every megaparsec it is away from us. So another galaxy that is 1 Mpc away will be moving away from us at about 80 km/sec, and one that is 10 Mpc away will be moving at about 800 km/sec.

There's a good write-up at the WMAP mission site entitled "How Fast is the Universe Expanding?" WMAP is scheduled to launch in the fall of 2000. One of WMAP's goals will be to accurately determine the Hubble constant to better than 5% accuracy.

Beth Barbier and Dr. Eric Christian

The Hubble Constant

To work out the age of the Universe, my teacher said that we have to divide 1 by the Hubble constant. 1/constant gives a constant, so how can this be? It seems like either the Hubble constant changes, or the Universe has a constant age.

The Hubble Constant is not really a "constant". As the age of the Universe changes, so does the Hubble constant. But on the timescale of human lifetimes, the change is much smaller than we can measure.

Dr. Eric Christian
(July 2002)


What is the Universe Expanding Into?

My understanding of the Universe is that it is constantly expanding in all directions -- the Big Bang theory. If this is true then what is it expanding into? How is the Universe creating more space?

This question was covered by Imagine the Universe! and by Scientific American.

Dr. Eric Christian

Speed of Our Galaxy

What is the speed at which our Galaxy is moving away from the theoretical point from which the Universe is expanding? What is the speed at which our solar system is moving around our galactic center?

A discussion on the "center of the universe" is on this site. We're moving at 600 km/sec relative to the cosmic background. You can find a good description of how we are moving through space at this NASA Astronomy Picture of the Day page.

Dr. Eric Christian and Beth Barbier

Center of the Universe?

I have seen the "expanding Universe" pictured as the surface of an inflating balloon. Using this image, can the direction back to the origin (center of the ballon) of the Universe be determined? Are there theories regarding what now occupies this point of origin? If there is nothing there, can we look through it to the far side of the Universe?

The inflating balloon example uses a 2-dimensional surface (the balloon's surface) to illustrate what is happening in 3 dimensions. The "center" of the balloon is not on the surface, and so if there is a "center" of the Universe, it is in the fourth dimension, not in the observable 3-D space. But a better way to think of it is that the whole Universe was at the center when the Big Bang happened (the balloon was scrunched up into a very small space). One has to be careful not to stretch one's analogies too far :-)

Here's a good illustration of this at UCLA

Dr. Eric Christian

Approaching Absolute Zero

The Universe gets colder because it's expanding, but it can't get to 0 ^o K, can it?

Absolute zero cannot be obtained, and the approach to it will be very, very slow.

Dr. Eric Christian

Dropping Temperature and Expansion of the Universe

We know that the temperature of the Universe is 3 kelvins and that, because it is expanding, the temperature is still dropping. Is it possible that when it reaches zero kelvins, the Universe will stop expanding because it'll "run out of energy?"

Although the temperature of the Universe will continue to drop, it will approach zero but never reach it. The energy that is in the temperature will spread out thinner and thinner, but since that energy is non-zero, no matter what volume it is spread over, the average is never exactly zero, just smaller and smaller.

Dr. Eric Christian
(September 2001)


When the Universe Cools

As the Universe expands, it cools. When the Universe gets really cold, will the entire Universe become a Bose-Einstein Condensate (BEC) and then collapse back to a singularity, closing the Universe?

When the Universe has cooled down to less than a Kelvin, BEC might form locally (inside former stars and planets), but there is no known force that is stronger than gravity across the vast distances that the Universe will have spread. So if gravity doesn't close the Universe, nothing we currently know of will do it.

Dr. Eric Christian and Beth Barbier