The Question (Submitted December 02, 1996) I'm a 10 year old boy who has a very hard question to ask. My teacher Mr. Sperling just had us learn about the Solar System. We just made a Internet site. Now my question. What is at the edge of the Universe? The Answer That's an interesting question.

The short answer is that there isn't any "edge" to the Universe, as in the edge of your school grounds where there is more property beyond. Science fiction and other dimensions aside, the best way of looking at the Universe is to think of the surface of a balloon. Right now the "balloon" is expanding (being blown up) so the distance between any two points on the balloon is increasing. However, there is no edge to the surface of the balloon. This is where cosmologists (people who study the physical nature and evolution of the Universe) and relativists (people who study Einstein's general theory of relativity) talk about a curved space-time continuum. One observable effect of this geometry for the Universe is if we look far enough in any direction, we see the same thing.

Because light does not travel infinitely fast, the farther into the distance that we look, the farther back into time that we look. In astronomy there is something called the cosmic microwave background. This radiation is left over from the "big bang", the event at the start (in time) of our Universe. The Question (Submitted November 24, 1997) I'm a 13 year old student from Denmark, who wants to know how big the Universe is and how the size of it is measured.

The Answer The simple answer is that the observable Universe is about 10 billion light years in radius. That number is obtained by multiplying how old we think the Universe is by the speed of light. The reasoning there is quite straightforward: we can only see out to that distance from which light can have reached us since the Universe began. (But see my note marked below). We determine the age of the Universe in a number of ways. One is to estimate the age of the oldest stars we see.

Our knowledge of how stars of a given size evolve with time is very good (based on what we know about atomic and nuclear physics) so the major uncertainty here is usually measuring how far away (and so how big) such stars are. The standard method is to look for very small changes in the apparent positions of the stars as the Earth moves around the Sun. (This effect is called parallax). A second way to get an age for the Universe is to try to figure out the time of the big bang itself. Here the method is to use a series of techniques (based on how bright things appear to be - like Cepheid variable stars - that we think we know the true brightness of) to determine first the distance of the nearby galaxies, then increasingly distant galaxies, until we have estimated distances for many galaxies for which relative velocity measurements have been made (using the Doppler red shift of features in their spectra). The relative velocities we observe for distant galaxies have been largely determined by the expansion of the Universe begun with the 'big bang'.

So, once we " ve determined how expansion velocity correlates with distance for some range of distances, it's possible to extrapolate back (with some assumptions) to calculate the instant of the big bang, when all the matter in the Universe was at a single point. (If any of these terms like 'parallax', 'Cepheid' and 'red shift' are unfamiliar, try entering them in the search window on our home page). The determination of greater and greater distances is one of the great themes of astronomy. Most introductory books will give you an outline of the story, which you can then fill in to any level of detail with further reading.

Our website has a lot of material on recent developments. For instance, there are already several answers in the 'Ask a High-Energy Astronomer' archive which deal with the size and age of the Universe. If you enter things like 'size of the Universe', 'age of the Universe', or 'distance scale' in our search window you will get lists of links to many of the most relevant discussions. Paul Butterworth for the Ask a High-Energy Astronomer team Note: The observable Universe may be only a small part of the physical Universe. In some theories, the Universe may have expanded very fast just after the 'big bang', and only a little bit may have remained within range of detection.

See, for instance: web > The Question (Submitted March 28, 1997) It seems that it doesn't make sense to think of the Universe as having either a center or an edge, but does it then make at all sense to think of it as having any kind of dimension or size? Perhaps the Universe contains all sizes and therefore has no sizes. Perhaps I've been reading too much about Richard Gott and this very bizarre de Sitter space. The Answer You are right that it doesn't make sense to think of the Universe as having an edge. It is better to think of the Universe as the surface of a balloon, with the stars and galaxies on the that surface. However, we can still think of the Universe as having size and dimension. Indeed, the Universe does contain all sizes, but it too has a size, just as the balloon has a size.

Physicists raise questions about the number of dimensions, and have theories for the Universe consisting of 11 dimensions. This, plus more familiar concepts such as the curvature of space-time, may make it difficult to think about the Universe having a size. But none the less, we consider that it does. The Question (Submitted September 22, 1997) What does the evidence tell us about the origin of the Universe and what is the evidence that concludes this point? The Answer The cosmological model of the origin of the Universe is a big subject, and I will only be able to give you a short sketch in this email, but there are many excellent books written by researchers for the general public that you can refer to. In 1912 American astronomer Ves to Slip her noticed that virtually every spiral galaxy he observed had a red shifted spectrum. The instrument he used split the light from the galaxies into a spectrum, in the same way a prism splits the light from the Sun into a rainbow.

Looking at light in this way, you can measure the intensity as a function of wavelength. Elements found in the galaxies each have "fingerprints": the spectral lines they emit. Since it is straightforward to measure the wavelength at which these spectral lines are emitted in different elements in the laboratory, looking at the spectrum of galaxies can give us a tremendous amount of information. In addition to seeing that the lines appear, astronomers can measure how far the wavelength we see them at differs from the "rest" wavelength. If the galaxy is moving, the lines will be Doppler shifted: they will appear at shorter wavelengths (bluer) if moving toward us, and at longer wavelengths (redder) if moving away. Edwin Hubble realized in the 1920 s that when we look at the motions of all of the galaxies, measured in this way, there is a definite trend.

The galaxies are speeding away from each other, consistent with a general expansion of the Universe. This was the first observational evidence to indicate an initial starting point of the Universe as a sort of explosion, from which everything is now still expanding. This is called the Big Bang. Hubble noticed that the measured recession velocity of a galaxy was proportional to its distance from us. This is called Hubble's Law, and the constant of proportionality is called the Hubble constant -- -the value of which is currently still a very active area of modern observational astronomy.

Hubble's Law has a very interesting implication for the history of the Universe. If we know how fast something is traveling away from us, it is a simple matter to calculate how long it has taken that thing to reach its present distance. If we assume the velocities of the galaxies we measure have been constant in time, then we can conclude that at a time = 1. / (Hubble constant) all the galaxies were virtually at the same location, starting their expansion. This time turns out to be about 13 billion years (using a value of the Hubble constant of 75 kilometers per second per megaparsec). It is this beginning that is known as the Big Bang.

The Big Bang theory has many predictions. In the 1940 s, physicist George Gamow realized that the very early Universe must have been very dense and very hot. As the Universe expanded and cooled down, this hot radiation should cool down, eventually being observable in the radio region of the spectrum. In the 1960 s Penzias and Wilson discovered the cosmic microwave background radiation: a uniform radio hiss that implied a temperature of about 3 degrees Kelvin. Later, the COsmic Background Explorer (COBE) took very detailed measurements of the spectrum and spatial distribution of this radiation, confirmed that it is extremely uniform, is of the spectral shape predicted by theory, and corresponds to a temperature of 2. 7 degrees Kelvin.

This observation provides strong support for the Big Bang theory. There are many fascinating branches of the story that I haven't even mentioned. Here are some references you should look at: S. Hawking "A Brief History of Time", C. Sagan, "Cosmos", S. Weinberg "The First Three Minutes." All are extremely readable, and written by great science communicators.

The Question (Submitted January 08, 1997) What holds the Universe together? What holds us together? The Answer Your very good questions are related to two different forces. The first, what holds the Universe together, is one that astronomers think about often. On large scales like the Universe, the most important force is gravity. Between any two objects the gravitational attraction is proportional to the product of the masses divided by the square of the distance between them. Gravity is the force responsible for keeping the Earth and other planets in our solar system in orbit around the Sun. Gravity also governs the motions of the Sun and nearly all the stars you can see in the sky, which are orbiting about the center of the Milky Way Galaxy.

The Milky Way is part of a gravitationally bound collection of galaxies which includes Andromeda, and is called the Local Group. Apart from observing that objects large and small are gravitationally attracted to each other, astronomers also observe that the Universe is expanding: an after-effect of the birth of the Universe in the Big Bang. Your second question, what holds us together, is closer to biochemistry than astrophysics. Human beings are composed of different types of large molecules: proteins, nucleic acids, lipids, carbohydrates, etc. These molecules are held together by intermolecular forces. An example is the peptide bond that links amino acids together.

This bond is formed when atoms of Hydrogen, Oxygen, Carbon and Nitrogen share electrons. Molecular bonding is governed by the electrostatic force, which on small scales is much stronger than the gravitational force for charged particles. We human beings still feel the effects of gravity though: it keeps us from floating off the Earth. The Question (Submitted March 27, 1998) Is there an area in the universe where cosmologist believe the Big Bang originated? Depending on where our galaxy (the Milky Way) is located in relationship to the origin of the Big Bang, might it not appear that the Universe were expanding or not? The Answer The question you have asked is a good one, and it involves concepts which are foreign to most of our everyday experiences. You may have heard something of Einstein's contributions to physics, in particular his theory of relativity. He introduced many unusual and counter- intuitive concepts that derived ultimately from simple, almost child- like thought experiments.

One of his contributions was showing that, in a Universe with no matter and no energy, time itself ceases to exist. Now, one corollary of this that comes into play as far is the Big Bang is its location. Some think of the Big Bang as a localized and very powerful explosion. This is not quite accurate.

All things that exist now or ever existed in the past were also present, although perhaps in different form, when the Universe was created. Therefore the Big Bang occurred everywhere all at once. You could not assign a location to it. The Big Bang was not so much an explosion really as the start of a great expansion, which continues even now. The rate of expansion would appear to be roughly the same, aside from local anisotropies, no matter where in the Universe you are. The Question (Submitted June 11, 1997) If all the distant galaxies are flying away from us, does that mean that we " re in the center of the Universe? The Answer Thanks for your question.

Astronomers and physicists interpret the result that all distant galaxies are flying away from us as evidence for the uniform expansion of the Universe. In this case, any observer, at any location in the Universe, observes the same general motion: that the further a galaxy is from us, the faster its relative velocity with respect to the observer is. The famous (and very illustrative) example of this is to imagine a loaf of raisin bread as it is baking. The raisins in the bread spread away from one another as the loaf rises and expands during the baking. Pick any raisin and pretend you are standing on it (you " re very small now! ) and measuring the rate at which the other raisins are moving away from you. You will find that, no matter which raisin you choose, all other raisins appear to be moving away from you, with the furthest raisins receding the fastest.

The current cosmological model of the Universe supposes that our position within the Universe is typical, not special. We are not located at the center of the Universe, but are rather taking part in its global expansion. I hope this answers your question. The Question (Submitted June 13, 1997) Can you tell me about the end of time? The Answer Thanks for your question about the end of time. In order to arrive at an answer, astronomers use their knowledge of gravity together with the Big Bang. We observe all distant galaxies to be receding from us, and from this we conclude that the Universe is expanding uniformly.

In fact, the current picture of the evolution of the cosmos is that since the birth of the Universe in the explosive Big Bang, the Universe has continued to expand. We also observe that massive objects attract each other through the gravitational force. This force tends to contract matter locally (for example, a gas cloud condenses to form a star). On the large scale you can think of the expansion of the Universe acting to separate galaxies from one another, and the gravitational force acting to attract them toward one another. The "end of time" depends on just how much mass there is in the Universe. We talk about this in terms of the density of the Universe, and compare densities to the critical density.

If the density is greater than the critical density, then eventually gravity will overtake the expansion. The expansion will slow down and eventually reverse, so that the Universe will be contracting. Eventually it will end in a collapse (or a bounce) called the Big Crunch. If the density is less than the critical density then the Universe will continue to expand forever, with the gravitational force never overtaking the expansion.

An ongoing area of research is to measure the density of the Universe. Currently, some observations (and some theories) indicate that the density of the Universe is very close to the critical density. In this case the expansion will slow down so that it is approaching zero expansion as time approaches infinity. If you are curious about this topic, you might want to check out the book "Cosmic Questions: Galactic Halos, Cold Dark Matter, and the End of Time" by Richard Morris (1993).

The Question (Submitted February 16, 1997) My questions relate to two matters that have been troubling me and for which I have not seen any comments by any astrophysicist or astronomer. The first question concerns the possibility for light emitted by a body that is 5 billion light years away from the earth, to survive for 5 billion years without being reduced to nothing during such long period of time. In other words, once light leaves its source it is no longer being fed with energy and thus it only dissipates energy through space and time. That being the case, how is it possible for such light to survive not only the distance but also the time.

The only explanation that makes any sense to me would be that which would hold that space is curved and that the distances we think we observe are nor real in a physical sense. Rather, they are relativistic. My second question relates to the theory of the big bang. If the theory is correct, then all of the observable Universe and beyond must be surrounded in a sphere of light that was created at the moment of the big bang. But because such sphere is expanding at the speed of light, we will never be able to observe it unless some of that light is reflected inwards towards the center of the sphere. Hence, if the Universe will eventually contract, the sphere of light will collapse back towards the point of origin and on its way there at the speed of light, it will illuminate (or burn up) everything in its path including our earth.

I do not really understand any of this but I would welcome the comments and views of an appropriate qualified individual. The Answer As for your first question, No, light does not dissipate its energy as it travels through space. It can only dissipate its energy if it interacts with matter. Light is a form of energy, and does not need to be "replenished" once it is it emitted. This is because light is actually made up of an electric field and a magnetic field which produce and support each other as the light beam travels through space. If you " ve ever seen an electric generator / motor you know that the coils of wires being spun inside the magnets can produce electricity (i.

e. an electric field). Also near power lines or motors compasses will become deflected because of the magnetic field produced by the electricity flowing through the power line or by the motor. It was in the 19 th century that James Clerk Maxwell discovered that a changing electric field produces a magnetic field, and that likewise a changing magnetic field produces an electric field. He also discovered that light was comprised of these changing electric and magnetic fields. Hence, a light beam is "self-sustaining." The only thing that stops this from going on forever is when a light wave interacts with some form of matter (ie.

a planet, dust, gas etc... ). Its energy is then absorbed by the matter. Sometimes the material may re-emit the light, but usually at a lower energy. Since space is mostly empty the chances of these waves encountering some matter is relatively small.

Hence, light can propagate outward for long lengths of time. Of course, as the waves spread out the intensity (or brightness) does gets weaker. This is because there are fixed number of waves spreading out into a larger area. This is known as "the inverse square law", because at any given point in space the intensity of the light decreases as the inverse square of the distance from the light emitting source. Nonetheless, its energy remains unchanged.

As for your second question, there is a residual effect from the Big Bang, and we can and have observed it. It's the Cosmic Background Radiation, which is observable in infrared wavelengths. Right after the first instant of the Big Bang, the energy was so great and dense that matter was constantly being created and destroyed (as predicted by Einstein's E = mc^2). The Universe was an expanding and cooling "soup" of energetic particles and photons.

Around a year after the Big Bang, the "soup" had expanded and cooled enough that the photons in the soup no longer interacted with matter. This left a "gas" of photons that has since expanded and cooled to 3 degrees Kelvin. This radiation permeates all of the Universe. The Cosmic Background Explorer (COBE) measured this radiation to an unparalleled precision.

For example, it has found that all but one part in 3000 of this "photon gas" contains energy from the Big Bang (in other words, the photons have essentially not interacted at all with the rest of the Universe since the Big Bang). You can learn more about COBE from web COBE has shown that much of the Big Bang is a good representation of how our Universe began and has ruled out some competing theories. We hope this helps you understand better these things that have been puzzling you. The Question (Submitted October 21, 2001) As a Philosophy student, I have linguistic tools to deal with space and time. These are bound intimately with consciousness. Having minimal experience with the astronomer's conception of space, and having only an amazed onlooker's idea of the "space" of the space of the universe, and having a child's wonder of the idea that the universe may be expanding unstoppably onward, then I ask a child's question: what is the universe expanding into? What "space" exists that allows us to say: this is big, and growing bigger into WHAT? Into what "space" is the universe expanding? And I am familiar with the idea that "expansion" and "growth" only happen within space / time parameters- philosophically speaking, reality is only apparent under the conditions that we "appear."..

so I have read that the "edge" of the "edge of the universe" idea really does not not hold true when one "does the math." I would like to know more about this math, that conditions a movement that expands, but expands not "into" not "out" or "toward" -anything else... The Answer Thank you for your question. Perhaps the simplest way to look at these questions is the following: if the universe includes, by definition, everything -- all of space, time, matter, energy -- than there can be nothing outside of it (and hence no edge), nothing for it to expand into. Its true that this is contrary to our everyday experience, as is much else in physics and astronomy; but of course our everyday experience does not extend to the entire universe.

In some ways this line of argument parallels those in refutations of the "argument by design" for the existence of God. Another way to look at it: if there were a higher-dimensional space in which the universe were embedded and into which it expands (like a two-dimensional balloon expanding into three-dimensional space), we could have no way of ever measuring the existence or characteristics of such a space. Whether such an unobservable space can truly be said to exist at all is a question best addressed by philosophers such as yourself! The Question (Submitted October 30, 2001) Given that the universe may have started from a singularity in a Big Bang, and that it seems that an awful lot of the universe is going to end up at the singularity inside a Black Hole, is it possible there is a connection between the two? I've seen the depiction of Space-Time as a rubber sheet with each mass causing a distortion to it - (recall Homer Simpson looking into the deep distortion caused by a Black Hole? ) - Is it possible for the singularity of a sufficiently massive Black Hole to distort Space-Time to the extent where it ruptures, the singularity exploding into the void beyond giving rise to another Big Bang and the start of another universe? The Answer Although the Big Bang also represents a spacetime singularity, it is not really a black hole. Actually, the Big Bang has more of a resemblance to the time-inverse of black holes: white holes (that may not actually exist in nature). But the Big Bang singularity is not really a white hole either -- there are technical differences in the natures of their event horizons and their connection to the rest of the universe and its constituents. It is also not necessarily the case that "an awful lot of the universe is going to end up at the singularity inside a Black Hole," since the universe could very well be Open rather than Closed and simply expand forever (in fact the latest evidence seems to point in that direction).

The Big Bang included all of spacetime and so does not represent a rupturing of some space in which it is embedded (that is, there is no "void beyond" the universe), and there is no danger of super massive black holes causing any sort of miniature Big Bangs although they are responsible for the super-energetic phenomena know as quasars. The Question (Submitted March 01, 1998) If it is possible to have hundreds of thousands of solar systems, is it possible to have hundreds of thousands of universes? The Answer People can, and are, going about extensive observing programs looking for planets around other stars, with a good deal of success recently. So not only can they exist, but we now know a few that actually do. On the other hand, there is no way to observe any universe other than our own. This is not a practical issue (like there not being good enough telescopes), but rather a fundamental theoretical issue. By definition, our universe is self-contained.

No other universe can affect anything in our universe, so we cannot gather evidence about its existence. Applying Einstein's laws of general relativity, it is possible to come up with solutions that connect one universe to another via a spinning black hole. However even if we were to observe such a thing, it would still not tell us if there was another universe on the other side or if the other side was part of our own universe. So, from a scientific standpoint the question is moot, but from a philosophical standpoint, it is fascinating. The Question (Submitted August 18, 1997) Hawking wrote about a singularity (physical as well as mathematical term), and the possibility that space begun its life from one of these. If space is expanding at speed comparable to the speed of light and if every galaxy is (approximately) considered to be homogeneous (speed of all material bodies within one galaxy is the same), would it mean that every galaxy has its own 'TIME' regarding to some reference time in spot of singularity? If so, possibility of parallel worlds would be reality.

The Answer Your question touches on one of the fundamental concepts of relativity. Observers moving relative to each other have their own 'time' in the sense that they may not agree whether two events happen at the same time. So there is no way to set up a single time system for the whole universe. Since the universe is expanding the relative speed of galaxies increases with distance. This means that there may be galaxies far enough away from us that the distance to them is increasing faster than the speed of light.

This might seem to conflict with Special Relativity but it happens because space itself is expanding. We are completely cut off from these regions of spacetime. There is no way to communicate with them because that would require us to send information faster than the speed of light. Maybe this is what you mean by parallel worlds, but they are not really parallel worlds. They are just regions of our universe that we can never reach or communicate with. The Question (Submitted February 11, 1998) What is the amount of energy released in the Big Bang.

Expressed in tons of dynamite or H-bombs, etc. The Answer Energy wasn't "released" per se - it's still contained within the event horizon, presumably. Notation: is an exponent - ie x 2 means x squared. "h is a multiplication symbol "h / is a division symbol The total mass-energy content of the universe today is of the order of the critical density, 3 x H 0 2/ (8 pi G) = 5 x 10 (-30) g / cm 3, times the volume contained within the present event horizon, (4/3) pi R 3, where R = the event horizon = c T (speed of light age of Universe) = 3 x 10 10 cm / s x (2/3) (c/H 0). Here H 0 is the Hubble constant, assumed to be around 50 km / s /Mpc and Omega = 1 (critical deceleration).

For this value of H 0, 1/H 0 = (app) 20 billion years, making the current age of the Universe about 2/ (3 H 0) = 13 billion years, so that R = (app. ) 1. 3 x 10 28 cm, which should be equivalent to 13 billion light-years (1. 3 x 10 10 y x 10 13 km / y x 10 5 cm / km ). This gives a total mass-energy mass of about 4.

4 x 10 55 grams, equivalent to about 2. 6 10 79 protons. The energy equivalent (E = m c 2) of these protons is about 2. 5 x 10 79 GeV or 2. 5 x 10 88 eV 1.

6 x 10 -19 J/eV = 4 x 10 69 Joules. One ton of TNT releases 4. 2 x 10 9 Joules. Thus the energy equivalent of the mass = energy of the universe is about 9.

5 x 10 53 Megatons of TNT. This is greater than the mass-energy of the universe, but only because the chemical process involved in exploding TNT is vastly less efficient that E = m c 2. The Question (Submitted January 19, 1998) I have heard that the universe is accelerating as it expands. Is that true? The Answer I think it is easy to get confused on this point. The universe is expanding, but the rate of expansion appears to vary with position. That is, more distant objects are receding from us faster than nearby ones.

The explanation for this is not that the more distant objects have accelerated relative to nearby ones. In fact it is just the opposite; when we view distant objects we look back in time, since light travels at a finite speed. That is, the expansion of the universe was faster at earlier times than it is right now. If we could see a true instantaneous snapshot of the universe at any given time (which is of course impossible) we would nearly uniform expansion. The Question (Submitted December 17, 1996) I'm not a rocket scientist, but I play one on TV.

Here are a couple of questions I've been pondering. 1. Is the Universe rotating? With reference to what? 2. Is dark matter necessarily within the bounds of the Universe? In other words, could something spatially disconnected from the Universe be having a gravitational effect on it? 3.

If you drew a circle of radius 1 billion light years, what would be the value of its circumference divided by its diameter (taking into consideration the curvature of space)? Same question but the largest circle that could be drawn in the Universe? 4. If the speed of light had been slowing since the big bang, would there be any way of knowing it? (Assuming that everything else would have been slowing with it. ) I think these fall into the category, "there's no such thing as a stupid question, just stupid people who don't ask questions." Thanks in advance for any light you can shed. The Answer Thank you again for your interesting questions. In addition to our Ask a High-Energy Astronomer staff at Imagine the Universe! , Demos Kazan as in the Lab for High Energy Astrophysics also contributed to these answers to your questions.

1. As far as we know, the Universe is not rotating. The presence of rotation would induce a type of change in the Cosmic Microwave Background temperature which has not been observed. In addition, the presence of rotation would imply that locations along the axis of the rotation were somehow "special", which violates our understanding of relativity that the Universe appears the same regardless of the location of the observer. 2. Yes, dark matter as we normally discuss it lies within the bounds of the Universe.

The effect of gravity is propagated by space-time, which is the fabric of the Universe itself. "Outside" the Universe there is no space-time, hence gravity has no effect. Remember that the best analogy for the "shape" of the Universe is that of a balloon being blown up. There is no "edge" to it. What lies "outside" it is undefined. 3.

It depends on the value of the spatial curvature, which in turn depends on the density of the Universe compared to the critical density to just halt the expansion of the Universe. If the Universe has any curvature (which results from the density being anything other than the critical density), then the circumference is larger than what it would be on a flat plane. The size of the effect does depend on the radius of the circle. To get a sense of scale, the farthest object that we can see to is ~ 20 billion light years away.

(Sorry we " re not giving you a specific numerical answer on this one! ) 4. This one caused us the most thought and reflection. I found a book entitled "Gravitation and Spacetime" by Hans C. Oha nian (1976, Norton & Co. ) to be helpful in settling some issues. We " ll first tackle the question you asked - whether a change in the speed of light could be detected.

The answer is, in principle, yes. One way is that the values for atomic transitions depend on the speed of light. We observe these transitions as lines in a spectrum. Hence, if the speed of light has changed, then the values for these transitions from sources far away (and hence which emitted their light long ago) would be different from present day values. The difficulty is that there are many other factors which cause the observed values of these transitions to change. These factors include the Doppler shift due to the expansion of the Universe, local motions of the object, gravitational redshifts, etc.

In practice, it would be difficult to disentangle the effect of a changing speed of light from these. Another way is to rely on a technique which utilizes the decay rates of various radioactive isotopes. These decay rates are very sensitive to the physical constants - the speed of light among them. If the values of these constants change with time, then nuclear "clocks" based on different isotopes would disagree. (A practical problem is figuring out which of the "constants" changed.

) Nonetheless, evidence shows that the ages of rocks measured by these different clocks agree quite well and hence puts tight constraints on the time variations of the physical constants. So in principle it can be done, and there is evidence that the physical constants are quite "constant." However, we should take this further and ask whether we should expect the speed of light to change with time. The speed of light is not just something we measure about light, but rather (as I've already implied) is a fundamental physical constant itself. Indeed, the constancy of the speed of light in space and time is an essential feature of special relativity. Time enters in because relativity treats time on an equal footing with space (hence the term, space-time). Consistent equations cannot be written if it is taken to be variable.

In addition, general relativity and our understanding of the evolution of the Universe is built upon the same premise. So my conclusion is that the speed of light is truly constant. You " ve given us a good workout. We hope our answers are helpful. The Question (Submitted February 10, 1999) Is the Universe's expansion rate slowing down or speeding up? I have always been taught that it was slowing down, but I read something in the New York Times that it was found about a year ago that it was actually speeding up. The Answer Up until a year or two ago, it was thought that the Universe's expansion rate was decreasing, due to gravity pulling back on the material exploding form the Big Bang.

However, more recently, scientists have been able to measure how fast the Universe was expanding, and the data indicate that it is actually speeding up. This measurement was made by looking at distant supernovae. If distant supernovae are different in brightness than nearby supernovae (e. g. if there is more dust dimming the light than we think) then the measurement could be wrong.

However, most astronomers think that the measurements are strong. The Question (Submitted August 21, 1998) I was listening to a few scientists talk on the radio about a force that is causing or assisting the expansion of the universe. I only had a brief moment to catch the radio show before having to go to work. They where saying that an anti gravity is causing the universe to expand. Is any one familiar with these ideas that may be able to point me in the right direction to get more information on this? The Answer You ask very good questions, that astronomers, physicists, and cosmologists are debating intently right now.

A group of researchers recently published findings that the universe may be expanding faster now than in the distant past. This would imply a non-zero value for the 'Cosmological Constant' (CC). Einstein's original cosmological model was a static, homogeneous model with spherical geometry. The gravitational effect of matter caused an acceleration in this model which Einstein did not want, since at the time the Universe was not known to be expanding. Thus Einstein introduced a cosmological constant into his equations for General Relativity. This term acts to counteract the gravitational pull of matter, and so it has been described as an anti-gravity effect.

see: web for a good but technical description of the 'force'. Until recently people just put a value of zero in for the (CC), now things may be different. We have yet to see what will hold up with newer instruments such as MAP. web coming online. The Question (Submitted February 12, 1999) Can two areas of the Universe be so far away from each other that the light from area A can not reach area B? The Answer It is now believed that the Universe is large enough that there can be areas A and B that have not been able to exchange light. This is a result of recent observations that indicate that the rate of the Universe's expansion is speeding up, contrary to what astronomers were expecting a couple of years ago.

The Question (Submitted June 30, 1997) I'm a college graduate with a degree in computer science. However, my favorite pastime has always been reading about astronomy, quantum mechanics, etc. that's my background. My question is: When astronomers speak of the estimated size of the "known Universe", are they setting this distance (from us) based upon the furthest visible object, or upon calculation? This is in reference to the fact that quasars (as far as I know) are the furthest observable objects.

Yet they travel at speeds approaching that of light away from us. Obviously, if there was anything further than the distance at which the expansion of the Universe = c, it would be impossible for us to detect it, now or ever. To sum up the question: how can one estimate the size of the Universe if any part of it past this critical distance is forever cut off from our measurement? One could argue that since we cannot ever reach these locations, for us they do not exist, but I think that's a horrible cop-out. The Answer What astronomers mean when they speak of the "known Universe" depends on the astronomer. Most often it refers to the region of the Universe from which light could travel to us since shortly after the Big Bang. The farthest observable discrete objects are the quasars (visible at such great distances because they are so bright).

However, the cosmic microwave background radiation, at 3 degrees Kelvin, comes from even further away. It has a redshift of about 1000, and comes from the time when the Universe was much smaller, and filled with hot ionized gas (plasma) at 3000 Kelvin, as hot as the surface of some stars. Dense plasma blocks light, and so we cannot see anything beyond that distance. If the theory known as "inflation" is true, the size of the "known Universe" is much smaller than that of the Universe as a whole.

If you look at the "known Universe", every part of it looks about the same, as far as we can tell. As an analogy, if you look at a typical cornfield in Kansas, it all looks the same as far as the eye can see. For there to be as much variety as you would expect in a world, the world has to be much larger than the size of a Kansas cornfield. Likewise, inflation says that the Universe is much larger than the known Universe.

How much larger is hard to determine, and theories are untrustworthy since we can never confirm them by observations. (Actually, 'never' is a bit of an overstatement. If you waited long enough, the Universe would slow its expansion and you may be able to see a bit further. But that would take billions of years. ) The Question (Submitted January 14, 1998) I have seen programs and read books about the Universe. Mainly they speak of it either being constant, never-ending or ever-expanding with the end of the universe happening when it finally collapses upon itself.

Could the "Big Bang" be a continuous cycle of collapse and expansion, collapse and expansion... ? It goes against everything in me to believe that one day it could end. I don't know how well I have stated my question, but I have always understood that matter is a constant. That paper can be burned and its composition changed, but the atoms are all still present only in a different form. I also understand that change is the only certainty in the universe as we know it, so an end does not seem possible. Has someone who has a respected opinion in this regard discussed it or disregarded this idea? The Answer I think this is a question that has occurred to many people contemplating the Big Bang.

Historically, cosmologists have fallen into 2 fairly clear categories: those who are very reluctant to contemplate a 'one-shot' universe (we might call them recyclers), and those who are not. Right now there is a preponderance of evidence that the universe will not collapse, based on measurements of the rate of expansion and the mass density. I agree with you that this is somewhat less appealing than recycling. However, I also am interested in how science often challenges my preconceived ideas, and so I am personally interested in keeping my mind open on this subject.

Die-hard recyclers will undoubtedly be able to suggest ways of bringing about collapse. The Question (Submitted November 13, 1997) I know that hydrogen is the most common element in the Universe. Can I know from you the known percentage of hydrogen in the Universe? And where it is precisely? The Answer When the Universe was formed in the Big Bang, the resulting elemental matter was about three quarters hydrogen, one quarter helium, and a few parts-per-billion of lithium (by weight). (When I say 'elemental matter', I am referring to matter made of the common chemical elements we see around us.

However, one of the great mysteries of astrophysics is, we don't know what most of the Universe is made of. Between 90% and 99% of the mass of the Universe seems to be completely unknown. This 'dark matter' has so far been detected only gravitationally: galaxies and clusters of galaxies seem to be heavier (or at least have more gravitational attraction) than can be explained by summing up all the stars and gas clouds we see. Some of this matter coalesced into galaxies and stars, although much remained as gas. As time went on, stars burned some of their hydrogen to heavier elements. These stars occasionally released their material (as winds, in supernova explosions, and in other events) so that it combined with the remaining gas, and formed new stars, and so on.

All material on Earth except for the hydrogen (such as that in water), helium, and lithium is this burned material. Our Sun, and most of the stars you can see in the night sky, are later-generation stars, and we can see the material burned by earlier stars in their atmospheres. Only a few percent of the original hydrogen and helium in the Universe has been burned this way. Most of it is still around, and so the elemental matter of the Universe is still about three quarters hydrogen, which is primarily in the form of clouds of gas and stars..