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

Tom Moran
Saratoga CA

If the velocity of galaxies is proportional to their distance, i.e., how long ago is the view we currently see, then it appears that long ago galaxies were receding rapidly, but today their velocity is much less. That sounds like an initial explosion with fragments losing velocity to gravity, with velocity loss proportional to length of time (constant deceleration). And if recessional velocity today is (ie, nearby) is near zero, does that mean the universe is near maximum size and about to start collapsing? That would seem surprising that we are observing just at that time.

What we see is a velocity of expansion that is proportional to distance from us. This is exactly what one expects for a uniform expansion. What is a uniform expansion? It is expansion where the length scale is increasing by a constant factor each second. For example, suppose you are in an auditorium which is doubling in size every 10 seconds (this would be an unusual room indeed!). After the first 10 seconds, a person who was originally 1 foot away is now 2 feet away - she appears to have moved 1 foot in 10 seconds or 0.1 feet/second. However in the next 10 seconds, she will go from 2 feet to 4 feet (0.2 feet/second), and in the subsequent 10 seconds she will go from 4 feet to 8 feet (0.4 feet/second). Thus you can see how this uniform expansion directly leads to a higher recession speed the farther away one gets. The universe is expanding in a very similar way. Now imagine that just when the room began to expand there was also a chair just 1.5 inches away. After 30 seconds, that chair would be 1 foot away. And thereafter, the chair would appear to recede just like the girl did.

Thus, the fact that nearby galaxies are receeding more slowly does not necessarily mean the expansion of the universe is slowing down. However, matter in the universe does indeed slow the expansion of the universe. It is generally believed that there is not enough matter in the universe to reverse the expansion and lead to a big crunch. But whether there is just enough matter to slow the flow to zero in infinite time (what we call assymtotically approaching zero expansion) is currently a matter of significant debate in observational cosmology.
M. P.

Name and City Withheld

Is it true that all galaxies appear to be moving away not only from our vantage point but from any/every given point in space? In other words,would not any point in space appear to be at the "center" from the observer's point of view? If true, how is this explained?

The answer is that the expansion is not like that of an explosion but it is what we call a uniform, isotropic expansion. What is uniform, isotropic expansion? It is expansion that looks the same no matter where you are. It is also an expansion where the length scale is increasing by a constant factor each second. For example, suppose you are in an auditorium which is doubling in size every 10 seconds (this would be an unusual room indeed!). After the first 10 seconds, a person who was originally 1 foot away is now 2 feet away - she appears to have moved 1 foot in 10 seconds or 0.1 feet/second. However in the next 10 seconds, she will go from 2 feet to 4 feet (0.2 feet/second), and in the subsequent 10 seconds she will go from 4 feet to 8 feet (0.4 feet/second). Thus you can see how this uniform expansion directly leads to a higher recession speed the farther away one gets.

Now imagine 3 people all standing at the corners of an equilateral triangle in our amazing expanding auditorium. Each person sees the other 2 moving away. Each person would also measure the SAME velocity-distance relationship (which in this case is V = (D/20) [V = velocity in feet/sec and D = distance in feet]). Each observer thus "sees" an expanding space with the same properties no matter where they are standing. Hence there is no one point that can be called the center. Even an observer far away from our 3 people would see their distance doubling from him every 10 seconds. If all of these people were asked to describe what they see, they would all say that they see the room doubling in size every 10 seconds.

Now how does this differ from an ordinary explosion? Well an observer at the center of the blast (if they survived!) would see other particles all moving away from him with roughly the same speed. However, observers who were flying away from the center (on the shock wave, for example) would not see the same flow as the central observer - particles close by would be moving away more slowly then particles far away. And observers who were far away from the blast may see some particles moving towards them and some moving away from them. Hence the velocity-distance relation that an observer measures depends greatly on where that observer is - the expansion is highly non-isotropic.

The concept of a center makes sense in the case of an explosion - it would be the point at which the observer sees all particles moving away with the same speed in all directions. However, as our expanding room demonstrates, when you have a uniform expansion, as is the case with the expanding universe, there is no center since all observers see the same flow no matter where they are.
M. P.

Patrick Wiegand
Staten Island, NY

OK:IF IVE GOT THIS CORRECT:THE UNIVERSE IS LIKE AN EXPANDING BALLOON MOVING IN A SET DIRECTION. NOW, IS THERE A WAY WE COULD LOOK AT OUR OWN GALAXY AT SOME TIME IN THE DISTANT PAST? IM SURE THIS IS AN OVERLY SIMPLISTIC QUESTION ...AND IF NOT, WHY ?

We cannot see our galaxy as it was way in the past. The reason we see back in time as we look farther out into space is that it takes longer and longer for the light to get here. We see the moon as it was about 1 second ago (it takes light 1.3 seconds to travel the 240,000 mile gap between the moon and the earth), we see the sun as it was 8 minutes ago (it takes light 8 minutes or so to travel the 93,000,000 mile gap between the sun and the earth). A light year is the distance light travels in space in 1 year. When we look at the center of our galaxy we see it as it looked about 24,000 years ago (because the Galaxy's center is 24,000 light years away). We see the Andromeda Galaxy as it was 2 million years ago. Hence, we can only view a given object at a given point in its history - a point which is equal to its distance from us in light years. Since our Galaxy is only about 50,000 light years across the farthest back in time we can look, in principle, is 50,000 years (and then only by looking at that part of the galaxy directly opposite our position - a nearly impossible task since it means looking through all the gas and dust that makes up our galaxy).
M. P.

Reginald Wilson
Rialto, CA

If Hubble should locate a galaxy at the edge of the visible universe,then shouldn't we be seeing the begining of the big bang? and if so,then how to explain more than one galaxy present at the begining of time? where does the initial singularity begin?

The Hubble will never see a galaxy at the very beginning of the big bang because galaxies did not exist at the very beginning. We think galaxies first formed about 1 billion years after the bang (but we're not absolutely sure of that). The next generation space telescope may be able to detect the very first galaxies.

Why were there no galaxies (or stars or planets) right at the start of the big bang? Because for the first few hundred thousand years, matter in the universe was so hot that it only existed as a soup of protons, neutrons, electrons, photons (light), and a few other subatomic particles. Hydrogen could only exist in an ionized phase (and elements like oxygen and nitrogen did not yet exist at all!). Under these conditions, a beam of light (what we call a photon) could not travel far before hitting one of the particles. When such a collision occurs the light is scattered in another direction (or in some cases, actually creates a new particle of matter). With the universe in such a state, one can only "see" things which are happening out to a distance equal to the typical path length between these matter-photon collisions - which at that time was a very, very small distance. It was not unlike being in a very thick fog. Thus when we look back at this radiation, we can only see a very small depth beyond it.

Then something wonderful happened - the universe cooled enough for neutral Hydrogen to form. When neutral Hydrogen forms, an electron gets bound around a proton and the electron is no longer free to roam space but must travel around the much heavier proton. The consequence of this is that at that point in cosmic history there were many fewer free particles to scatter the light and space began to clear - the fog lifted, if you will.

This event in cosmic history puts up a barrier through which no telescope can see no matter how powerful or big it is (cosmologists refer to this barrier as the "surface of last scattering"). Hence, we will always be limited to seeing back to the time when the universe was just a few thousand years old. This is an interesting time though - there were no galaxies, stars, or planets then and we learn a lot about the way the matter in the universe was distributed before all these objects we now know and wonder about were even formed. It is hoped that the next generation space telescope may even detect galaxies at the earliest moments of their formation - well after the "epoch of last scattering" but a billion years earlier than we can currently see with the Hubble Space Telescope.

However, there is one way to get past this cosmic barrier - and that is to create the very high energy conditions which existed in the very early universe right here on earth! Inside huge particle accelerators (e.g., at Brookhaven or Fermilab) we can create conditions similar to what was happening when the universe was much younger than 100,000 years. And that is why we think we know what the universe was indeed like shortly after the Big Bang.

As for when the bang occurred, the current best estimates are it happened sometime between 12 and 16 billion years ago.
M. P.

Name and City Withheld

If there is other universes beyond our own,then could it have followed different physical laws than those we know? and if so, could our understanding of the laws be totaly wrong?

There could be other universes and they could have very different physical laws. The problem is that our understanding of physics in OUR universe is not sufficient to allow us to describe what occurred before the first micro-micro-micro-micro-micro-micro-micro second (10^-43 second) of the universe's existance.

Before this time, the universe was so small that it actually was comparable in size to the scale over which quantum forces act and the current assumptions made in physics break down (normally we can describe the universe once it becomes larger than this quantum scale or what physicists refer to as the Planck scale [named after one of the pioneers of modern quantum mechanics]).

Hence until we make a major breakthrough in understanding the universe when it was smaller than the Planck scale, we will be limited to only speculating about the existance and nature of other universes.
M. P.

Ron Holland
Davidsonville, Maryland

Dear Ladies and Gentlemen:
Please tell me if a photon has mass. This has been a question that I have not been able to get answered since my college days in the 60's. Thank you.

No, a photon does not have mass but it does have energy. One of Einstein's great contributions is his theory that energy and mass are related. Hence, although the photon has zero mass, its non-zero energy can (under some conditions, like those in the very early universe) be used to convert the zero-mass photon into a pair of particles which do have mass.

The photon's path can also be altered by the presence of a large mass, like a star or galaxy. However, this is not because the photon is being "attracted" by the star but because the star has caused the space around it to be curved. Since photons are confined to travel in paths which exactly mirror the curvature of space, if a star or galaxy distorts space then photons passing near by will follow curved paths instead of straight lines.
M. P.

Erik Silberbauer
Ossining, NY

Fantastic Program, extremely interesting!

Please help me understand how we can see back in time. In order for us to see light emanating from the time of the big bang, we would have to have travelled faster than light and evolved to what we are in order to view light from that time.

I would appreciate a response or a direction to go in an effort to clarify this point. Thank you.

The reason we see back in time as we look farther out into space is that it takes longer and longer for the light to get here. We see the moon as it was about 1 second ago (it takes light 1.3 seconds to travel the 240,000 mile gap between the moon and the earth), we see the sun as it was 8 minutes ago (it takes light 8 minutes or so to travel the 93,000,000 mile gap between the sun and the earth). A light year is the distance light travels in space in 1 year, which is approximately 6 trillion miles. When we look at the center of our galaxy we see it as it looked about 24,000 years ago (because the Galaxy's center is 24,000 light years away). We see the Andromeda Galaxy as it was 2 million years ago. Hence, we can only view a given object at a given point in its history - a point which is equal to its distance from us in light years.

We believe the big bang occurred about 12 to 16 billion years ago. Hence, light from that time would take 12 to 16 billion years to reach us.
M. P.

I recently shared the subject matter of the PBS special with a friend...he said that he had read something somewhere about a galaxy that is moving in the wrong direction relative to the big bang. Do you know anything of this?

Also, I was told recently that the square of Pegasus is relatively SUPER rich with galaxies. Can you confirm this?

The motions of all galaxies are comprised of two main components. The first is the motion due to the expansion of the universe. The second is a motion induced by the mutual gravitational forces between it and its neighboring galaxies. For very distant galaxies, the expansion of the universe is the dominant motion (at least from our vantage point). But for some nearby galaxies, the motion generated by the gravitational pull from another close galaxy or galaxy cluster can exceed its expansion velocity. For example, the Andromeda galaxy - a galaxy close in size to our own which is a mere 2 million light years away - is actually falling towards us rather than moving away. This is a result of the gravitational attraction between Andromeda and the Milky Way galaxy.

There are a number of "superclusters" - associations of thousands of galaxies which can span an area 30 million light years across - which are massive enough to cause lots of galaxies in their vicinity to have gravitationally induced motions that compare with or even over-power their expansion motion. Some of the more prominent nearby superclusters lie way beyond the stars which comprise commonly known constellations. There are such superclusters in Virgo, Perseus, Pisces, Hydra, and Centaurus to name a few.
M. P.

Steve Steinberg
Margate City, New Jersey

Excellent first show! I Am looking forward to the remaining shows.

I imagine that time on the Hubble telescope is probably reserved until the year 2010 or thereabouts, but has any thought been given to combining the very large array radio telescope with the Hubble? In the past decade there has been at least one highly publicized contact on the radio telescope. It might be rewarding to point Hubble (or Hubble 2) in the same direction of the universe to see if the source of the contact can be determined.

Actually the Hubble Space telescope is only scheduled about 1 year in advance. This is actually good since it means the people using the telescope will always be pursuing the best and most interesting science.

Hubble has in fact been used (and will continue to be used) in conjunction with radio telescopes, infrared telescopes, x-ray telescopes, and even amma-ray telescopes. You are correct that combining the sharp, optical images from Hubble with data in other wavelength regions can provide important reakthroughs in our understanding of radio or x-ray sources in space.

The Hubble, for example, was used very recently to observe an area of space where a gamma ray burst was detected. Gamma rays are very high energy photons and the telescopes which detect them have only crude estimates of where in the sky the photons came from. By combining Hubble data and data from another x-ray telescope, we were able to pinpoint the probable source of the gamma rays.
M. P.