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The Universe

This page contains answers to questions about the Universe. The questions are:
[133] What happens to the incredibly vast quantity of energy that is radiated by the billions of stars in our galaxy? Will this not tend to raise the average temperature of the interstellar medium? If not how does one reconcile the Law of Conservation of Energy when applied here?
[132] What is the average intergalactic temperature?
[116] Can we still see the light of the Big Bang?
[70] How many atoms are there in the Universe? What is the most common atom in the Universe?
[69] What are the mass, size, and density of the Universe?
[42] Is the Universe a closed finite system (it had an origin point, and it will stop growing and maybe die like stars or humans)?

For structured information about what you can find in the Universe, see the Universe Tree Page.

The Expanding Universe

According to the Big Bang theory (which has a lot of support from the observations), the Universe started very small sometime between ten and twenty thousand million years ago and has been expanding every since. The expansion of the Universe has no particular center point: it grows by gaining space everywhere between stars and galaxies. You can compare it to a balloon with dots on it that is being inflated: All the dots get further and further away from each other, but there is no one place on the balloon that is the center of the expansion. From each dot it appears as if all other dots are moving away from it, and the further away a dot is, the faster it appears to move away. Currently, space grows at a speed of between one and two parts in 20 thousand million every year, so a distance of about 3 million miles in space between the galaxies grows by one foot every year, and a length of about 15 million kilometers somewhere between the galaxies grows by one meter every year.

Only the space between things is increasing: not the size of things themselves. If everything in our Universe were growing at the same rate, then our measuring sticks would grow at the same rate as the rest and we would not be able to measure the growth.

The Universe is certainly closed in the sense that we can only observe things out to a certain distance. The Universe is about 15 thousand million years old (plus or minus 5 thousand million years or so), so light from places that are further away than 15 thousand million lightyears has not yet been able to reach us. When we look at things that are very far away, then we see them as they were when the light started on its journey from there to here, so looking far away is the same as looking into the past. For example, when we observe something at a distance of 14 thousand million lightyears, then we see it as it was only 1 thousand million years after the Big Bang (assuming an age of 15 thousand million years for the Universe). The largest distance from where light has had time enough to reach us grows at the speed of light, so one might say that the visible Universe grows at the speed of light.

There may very well be things beyond the current edge of visibility of the Universe, but we cannot know anything about them because no information from beyond the edge has been able to reach us. We cannot know if the Universe beyond what we can see has a finite size or not, if it is closed or not.

The Slowing Expansion of the Universe

The expansion of the Universe is slowed down by the gravity of all the mass that is in the Universe. Because the Universe is growing, the density of the mass in it is decreasing and the gravitational slow-down is itself slowing down. If there is enough mass in the Universe, then gravity wins and the Universe will some day stop growing and start to shrink again until perhaps a final "Big Crunch". If there is not enough mass, then expansion wins and the Universe will continue growing forever. The mass density just between these two cases is called the "critical density". Either way, life in the universe will become impossible at some point, but this time is still many thousands of millions of years in the future.

Our best estimates of the mass density in the Universe are quite close to the critical density. The lowest estimates are low enough that the Universe keeps expanding forever, and the highest estimates are high enough that the Universe will start shrinking at some point, so at the moment we cannot tell what will happen to the Universe. We cannot tell if the Universe is open (i.e., ever expanding) or closed (i.e., has a finite life time) in the time direction.

This uncertainty about the fate of the Universe is one of the reasons that astronomers are so interested in the so-called "dark matter". They have indications that a large part of the material in the Universe may be invisible (dark) so that we have missed a lot of material in our estimates of the mass density of the Universe. If the presence of a large amount of dark matter can be either proved or disproved, then we can sharpen up our estimates of the mass density in the Universe and then we may be able to figure out the fate of the Universe.

Mass, Size, and Density of the Universe

The mass, size, and density of the universe involve very big and very small numbers with large numbers of zeros in front of or behind the decimal point. Scientists write such numbers using a special code that keeps them short. When they write a number like 6e7 then the number after the "e" indicates by how many places the decimal point must be shifted to the right. For instance, 6e7 is equal to a 6 with 7 zeros behind it, or 60,000,000, and 6.1e7 is equal to 61,000,000. If the number after the "e" is negative, then the decimal point is shifted to the left. For instance, 6e-3 is equal to a 6 with 3 zeros in front of it (and the decimal point after the first zero), or 0.006, and 6.1e-3 is equal to 0.0061.

The mass density of visible matter (i.e., galaxies) in the Universe is estimated at 3e-28 kg/m^3 (3e-31 times the mass density of water). The radius of the visible Universe is estimated at 1.7e26 m (18 thousand million lightyears) plus or minus 20 percent or so. This yields a total mass of the visible matter of about 6e51 kg (1.3e52 lb), which is equivalent to the weight of 4e78 hydrogen atoms. Since nine out of ten atoms and ions in the Universe are in the form of hydrogen, this is a reasonable estimate for the number of atoms in the Universe (based on the visible galaxies only). Maybe a correction factor of the order of 2 has to be applied to account for the warping of space on very large scales.

However, there is considerable uncertainty about the mass density of all matter (visible and invisible) and energy (through Einstein's E = mc^2 equation). When one studies the movement of matter in and around galaxies, then it appears that up to about 10 times more mass is pulling at the matter (through its gravity) than is accounted for in the visible stars. This is the "missing-mass" problem. If this factor of ten holds throughout the Universe, then the total mass in the Universe would be about 6e52 kg. If the missing mass were mostly in the form of hydrogen atoms (which is not at all clear) then the number of atoms would be about 4e79.

A currently popular theory of the formation of the Universe (the so-called Inflation Theory) predicts that the mass density of the Universe should be close to the so-called critical density that separates an open universe that always grows from a closed universe that ultimately collapses again. This critical mass density is currently equal to 6e-27 kg/m^3. If the Universe is at the critical density, then the total mass of the Universe is closer to 1e53 kg, and the number of atoms (assuming that most of the mass is in the form of hydrogen atoms) about 6e79.

It seems, then, that the number of atoms in the Universe is at least about 4e78, but perhaps as many as 6e79. I would suggest 1e79 as a reasonable estimate. That is, 10 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 atoms.

Of course, besides material particles there are also lots of photons and neutrinos flying around the Universe. It is estimated that there are about 1e9 times as many photons and neutrinos as atoms in the Universe.

[LS 1 April 1997]

Intergalactic space

Although intergalactic space is fairly empty, one thing that is definitely there is the cosmic background radiation. This ubiquitous radiation is a residue of the Big Bang that is still detectable at microwave wavelengths. It was discovered, almost by accident, in 1965 by Arno Penzias and Robert Wilson at the Bell Laboratories. Penzias and Wilson have since been awarded the Nobel Prize in Physics for their discovery.

The Cosmic Background Explorer (COBE) spacecraft has now measured the energy distribution of this background radiation very accurately. It is an astonishingly good match to a theoretical black body distribution for 2.735 ± 0.06 kelvin, that is, 2.735 degrees above absolute zero. This is also probably the best number for the intergalactic temperature.

There are other things, such as dilute starlight, cosmic rays, and neutrinos, in intergalactic space that are not in thermal equilibrium with the microwave background. In the very imperfect sense that we can talk about the temperature of these other things at all, they are much, much hotter than the microwave background, and represent a minor, but annoying, qualification to my value for the intergalactic temperature given above.

[RR 15 April 1998]

Where does the stellar energy go?

The radiation coming from stars increases the amount of energy in the universe outside of stars and heats up whatever material absorbs it, but only a fraction of that radiation is absorbed by material in our galaxy. A simple model (based on numbers for galactic absorption listed in Allen, "Astrophysical Quantities" -- Athlone Press, London, 1973) suggests that on average at most about half of the visible light emitted by a sun-type star is absorbed in the galaxy. Infrared light and radio waves have a better chance of escaping, and UV light has a smaller chance, so of radiation emitted by the vast number of cooler objects in our galaxy (such as planets, gas clouds, and very cool stars) a smaller fraction is absorbed before it can escape from the galaxy. For instance, of light emitted by a typical object in the galaxy with a surface temperature of 1400 K (a quarter of that of the Sun), only about 10 percent is absorbed.

In addition, the absorbed radiation is spread over an immense volume (corresponding to about 600 cubic lightyears per solar luminosity in the galactic neighborhood of the Sun), so that all of this absorbed starlight would heat up extremely cold interstellar material to a temperature of only about 3.1 K (-270 degrees Celsius, -454 degrees Fahrenheit). For room-temperature objects the temperature increase from starlight would be only about one millionth of a degree.

Of course, starlight is not spread evenly across the whole galaxy. Some places receive more starlight than average (for instance, the Earth), and other places receive less. Halfway between the Sun and the next nearest star, alpha Centauri, the equilibrium temperature would be only about 1 K (including the radiation from both the Sun and alpha Centauri).

The starlight that is absorbed heats the receiving object up a little so that that object starts radiating more itself, until the extra received radiation is balanced by the extra emitted radiation. Not even black holes can store received energy indefinitely (and the fraction of the galaxy's radiation that is absorbed by galactic black holes is negligible anyway). Ultimately, then, all the energy generated by the stars in our galaxy leaves the galaxy.

One might therefore expect that the intergalactic radiation density must be increasing all the time, but this is not the case. The universe is growing in size (as expressed by the Hubble law), so the intergalactic radiation gets spread out over an increasing amount of space and its density actually decreases with time. This is fortunate, because just after the Big Bang the universe was so hot that ordinary material (atoms) could not exist. If the universe had not expanded, it would still be that hot (following the law of conservation of energy) and no stable materials (including that from which the Earth, stars, and we are made) would exist. Much of the "hot" radiation from those days is still around, but at a vastly lower temperature of 2.7 K. This is the so-called "3-kelvin background radiation" (see below).

[LS 22 April 1998]

Residue of the Big Bang

The radiation that resulted from the Big Bang is still detectable. It was generated when the universe was very hot, but because of the expansion of the universe it has cooled down to a temperature of 2.73 kelvin (270.42 degrees Celsius below zero) and is commonly referred to as the "3-kelvin background radiation". The radiation is strongest at a wavelength of about 1 mm (in the very far infrared) and comes almost equally from all directions. It was discovered by accident 30 - 40 years ago.

[LS 7 December 1997]

Additional Reading

You can read more about these things in "In Search of the Big Bang: Quantum Physics and Cosmology" by John Gribbin (Bantam Books, New York, 1986: ISBN 0553342584 (paperback)).

[LS 1 November 1996]

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Last modified 30 April 1998

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