Ask Us

Space Physics:
Cosmology - Properties of the Universe

The Fabric of Space

Assume that the presence of matter curves space, and that space has some sort of a structure to it. If the Earth, for example, did not experience a push from the fabric of space, which would create a tension in the fabric of space, would it fall in space? This would be applicable to all objects in the Universe. If there was a push or tension in the fabric of space, would this mean that the Universe is bounded, and there is some sort of force that is keeping space in tension?

Although it is true that matter does curve spacetime, that is really only an analogy that should not be taken too literally. While imagining the Earth as a bowling ball on a rubber sheet does provide a good visual representation of how objects and light move with respect to a more massive body, it is important to realize that this is a two-dimensional view, and that the "tension" on that sheet is not pertinent to the Universe.

The question of whether or not the Universe is bounded is somewhat different. It refers to the relationship between the amount of matter in the Universe and the gravitational pull that all objects exert on one another as they move away from one another.

Lauren Scott
(March 2003)


Age of the Universe

Has the age of the Universe been pinned down yet? Is it still thought to be between 10-20 billion years old?

This is one of the main questions being addressed by the WMAP team. Check out their web page on Cepheid Variables.

Dr. Louis Barbier

How Will the Universe End?

What will be the end (death) of the Universe?

Please see our sister Web site, Imagine the Universe! for a good discussion of this.

Dr. Louis Barbier

Uniformity of the Universe

My astronomy teacher told our class that the Universe is overall homogenous, and that the Universe is expanding. He also said that scientists have trouble figuring out why the Universe is so uniform in all directions. I also know that there are three major theories of the shape of the Universe: spherical, Euclidean, and saddle Universe. If the shape is spherical, is it possible that the reason the Universe looks the same when we're looking to the north as it does when we're looking to the south is because the light bends in such away that we're actually looking at the same thing?

It's not that the Universe is so uniform that is the puzzle. It's the fact that the remnant of the Big Bang (the cosmic microwave background) is very uniform, yet galaxies developed very early in the Universe. Galaxies are large non-uniformities in the mass of the Universe, and how matter got so clumped from such a smooth origin is the puzzle. While it is not impossible that light wraps around the Universe, there is no evidence that is does (we don't see the same galaxy or quasar in different directions).

Dr. Eric Christian

Temperature of the "Void"

Is it correct that objects in deep space (not heated by an external source, such as the Sun) have a temperature near absolute zero - but the void itself has NO temperature?

Objects in the void will eventually come to the equilibrium temperature of the cosmic microwave background, which is 2.7 Kelvin (2.7 degrees above absolute zero). The cosmic background is a sea of photons (light) that are the remnants of the Big Bang, which has cooled down to 2.7 Kelvin over 15-20 billion years. The comment that the void itself has no temperature comes from the fact that temperature is usually defined with the random motions of matter, and if there is no matter, there is no temperature. But I (and many others) equate the cosmic background with the 'void' having an effective temperature of 2.7 Kelvin, even if there is no matter.

Dr. Eric Christian

Conservation of Energy and Big Bang

How are the big bang and the law of conservation of energy compatible?

Since the Universe (as far as we can see) has a finite amount of energy, all that is required is for that amount of energy to be present in the Big Bang, and energy is conserved. There is no way to prove that this is true, but it is a good working hypothesis.

Dr. Eric Christian

Big Bang and Nucleosynthesis

How is cosmic nucleosynthesis and helium abundance related to the Big Bang theory?

Most of the helium in the Universe was created about three minutes after the Big Bang, when the temperature had cooled enough for nucleosynthesis to take place. Current theory says that about 25% of the baryon mass would have been helium after the Big Bang. Only 1 or 2% more helium has been created in stars since then.

Dr. Eric Christian

Formation of Heavy Elements

Do heavy elements like uranium form in supernovas?

Uranium is only formed in supernova explosions. There are other heavy elements that can be formed in the cores of supermassive stars. Heavy is usually defined as all of the elements above nickel (element 28) which require energy for their fusion, instead of giving off energy (as nickel and lighter elements do).

Dr. Eric Christian
(March 2002)


Formation of Gold

What stellar process is needed for gold to exist?

Gold and the other heavy elements (heavier than nickel) are made through one of three processes. These are named the r-, s-, and p-process. r stands for rapid, s for slow and p originally stood for proton capture (though it turns out that is not the dominant mechanism).

The slow and rapid refer to the time scales for neutron capture, versus nuclear beta decay. It's not possible to give the details of these mechanisms here; I suggest you refer to a textbook. These processes typically take place in association with supernovae.

Gold in particular is made by both the s- and r-processes, but mainly by electron emission from unstable r-process elements (e.g. hafnium).

Dr. Louis Barbier
(March 2002)


Composition of Molecular Clouds

I'm very interested in astrochemistry, particularly the composition of interstellar molecular clouds. Where can I find more about the chemical abundances of elements and compounds in the molecular clouds discovered in space?

Thanks for your very interesting question. As you probably know, the composition of molecular clouds is rather similar to that of the Sun: 90% hydrogen, 9.99% helium and the rest in the form of heavier elements. There have been some interesting discoveries of the chemical composition of clouds, such as some organic compounds and mixed-molecular ices. We recommend a few helpful websites that focus on the study of molecular clouds. Please see the Large Millimeter Telescope's overview of Interstellar Chemistry and the NASA Ames Astrochemistry Lab's page on The Identification of Ices and Other Materials in Dense Interstellar Molecular Clouds.

Dr. Georgia de Nolfo
(April 2005)


Maximum Size of the Universe?

Since red shift is a measurement of how distant an object is, and the speed of light limits the maximum red shift, does this mean that the speed of light imposes a maximum size for the Universe?

The answer to your question is yes and no. The red shift is really not a factor, but the speed of light does impose a maximum size on the OBSERVABLE Universe. By this, I mean that if there is something furthur away than the speed of light times the current age of the Universe, the light will not have reached us yet, and so we can't know anything about it. That doesn't mean that the Universe isn't larger than we can see, we just can't prove whether it is or not.

Dr. Eric Christian

Motion of Galaxies

Are there any galaxies that aren't moving outward, but inward or in any other direction? If there are, doesn't this prove the Big Bang theory wrong?

The motions of galaxies have been changed by gravity over the billions of years since the formation of the universe. These are called peculiar motions and don't change the fact that "on the average" galaxies are moving away from us, and the farther they are, the faster they are moving. That is all that is implied by the Big Bang. Not every galaxy has to be exactly moving away from us at an exact speed.

Dr. Eric Christian
(May 2002)


Dark Matter

It's my understanding that 90% of the atoms in the Universe are hydrogen, 9% are helium. The remaining 1% are the heavier elements (made by stars, supernovas, etc). I wonder how 90% of the mass of the Universe can be "invisible" - unless the visible, measurable Universe accounts for just 10%. Another Web site told me that that was indeed the case. (The supposed percentage of invisible stuff changes each week, but it's still pretty high.) They said that those percentages only concern what's visible.

I don't see how this can be, even if it's true that neutrinos have a bit of mass. How come only 10% of the Universe has been manipulated by gravity so as to have the present outcome - galaxies, etc.?

I'm afraid that I can't answer the "How come?" part of your question, because we don't know why the Universe is the way it is. But the observations seem to indicate that only about 10% (I agree that the numbers are variable in time) of the mass of the Universe is composed of baryons (protons and neutrons, i.e. atoms).

Whether the rest of the mass is neutrinos or something stranger (superstrings, neutralinos, WIMPs, or whatever) we don't know. But all of these objects have mass, and so they do interact gravitationally. That is how we know something is there: when you look at the gravitational field of galaxies, galactic clusters, and galactic superclusters, it appears that there is a lot more mass than can be explained by what we see in the stars, and the larger the length scale, the bigger the discrepency. This so-called Dark Matter is one of the big puzzles of Astrophysics. There is more information on Dark Matter at our sister Web site, Imagine the Universe!.

Dr. Eric Christian

Hubble and the Age of the Universe

There are independent studies that have used Hubble data to determine the age of the Universe to be roughly 14 billion years (give or take 500 million years). However, I read in articles that one of the studies made calculations based on observations of burnt-out stars that were 12 to 13 billion years old, plus 1 to 2 billion years for star formation. Don't stars take more than 1 to 2 billion years to form, to use up their fuel, and then to become a burnt-out star? Shouldn't this estimate be quite a bit longer? In other words, if they observed light from a burnt-out star that took 13 billion years to get here, wouldn't it have take that star 10 to 15 billion years to form, evolve, and burn out? Shouldn't we be talking about a Universe that is 20 billion years old or more?

The stars that Hubble looked at are in the Milky Way (our) Galaxy and so are within 100,000 light years of us. So the time line is 1-2 billion years for the stars to form, 12-13 billion years for them to burn out, and then less than 100,000 years for the light to get to us.

Dr. Eric Christian
(April 2002)


Edge of the Universe?

My son says that everything has a limit, an edge, and that the Universe should too. He believes the edge of the Universe to be where dark matter begins. Please help me to shed some light on this for him and for myself.

There is an edge to the Universe that we can see (what is called the "observable universe"), but we don't know if that's really the edge of the Universe. Because the Universe has an age (13.7 billion years old, from the NASA WMAP mission), and it takes time for light to travel, we can't see anything further away than 13.7 billion light years. Whether this is because there isn't anything there or there is something there and the light from those objects hasn't reached us yet, we can't tell.

Dark matter is a separate topic. We can tell from the spinning of galaxies and the orbits of galaxies in galactic clusters that there is more matter than we can see. And there are other, more complicated ways that tell us that this matter isn't made of the protons and neutrons that make up most the Universe that we can see. So scientists think that there is a lot of matter than has gravity but doesn't give off any kind of light, which is why we can't see it. This is what is called "dark matter".

I will have to find a way to simplify it for him, since he is not yet 6 years old.

To help you explain it, here's an analogy:

The horizon of the Earth is an edge, beyond which we cannot see things, and the Universe has an edge like this. In the case of the Earth, we can travel to the horizon and see what's beyond. But the edge of the Univese is very, very far away, and we can't travel there, so we don't know if anything is beyond it.

Dark matter is matter that we can feel (gravity) but not see. And we think it's all around us, but because it's so dark, we haven't found a way to measure it yet.

Dr. Eric Christian
(July 2009)