The Astrophysical Hotseat

[Borodog]

You may fire when ready.


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[Werebear]

What do you think of when you see pictures of Nancy Reagan naked?

[Borodog]

Send me one, and I'll let you know.


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[Samadhi]

Why is the expansion of the universe accelerating?

(thought I'd start easy )

[This message has been edited by Samadhi (edited 11-19-2002 01:56 PM).]

[SaberKitty]

Who said the universe was expanding? i thought recent calculations of omega were at 1.00 +/- .01

[Bicho the Inhaler]

I was under the impression that the universe was expanding.

[Samadhi]

Damn it! Why am I always the last to know?


Okay, then is the universe expanding or not?

And another question
Why do galaxies rotate with uniform angular momentum (not sure if that's the right term, you know why do they rotate like disks instead of whirlpools)?

[firemeboy]

For years people thought we were at the center of the universe. Then we decided we're not. Now it looks like everything is expanding away from us at the same speed. Can you clarify this without saying 'the curve of space-time'?

[Borodog]

Samadhi: Why is the expansion of the universe accelerating?

We don't know.

SaberKitty: Who said the universe was expanding? i thought recent calculations of omega were at 1.00 +/- .01.

The Universe is certainly expanding. Omega is merely the ratio of the actual density of the universe to the so-called critical density that would result in the eventual (at t=infinity) halting of the expansion of the universe (due to its own self-gravity), according to the standard cosmological theory. If Omega<1, the Universe would have exploded into empty nothingness long before stars and galaxies could have formed. If Omega>1, the universe would have almost instantaneously collapsed under its own gravity after the Big Bang. So it's actually not too surprising that we observe Omega~1.

Samadhi: Why do galaxies rotate with uniform angular momentum (not sure if that's the right term, you know why do they rotate like disks instead of whirlpools)?

The do not rotate like "disks", if by disks you mean like phonograph records. Nor do they rotate with uniform angular momentum. However, they do rotate strangely, with very interesting implications.

An object (say of mass m) in circular orbit about an object much more massive than itself will move at a velocity determined only by the mass of the central object and the distance from it. This velocity is found by setting the centrifugal acceleration required to move the object in a circle of radius r (mv^2/r) equal to the gravitational force proveded by the central object of mass M (GMm/r^2). The mass of the small object cancels, and you are left with the Keplerian velocity, V_k=sqrt(GM/r).

Thus, object moving around a massive point mass should have rotation curves that fall off as the inverse square root of the radius. We see planets in our solar system behaving in this fashion.

Spiral galaxies are flat, with their matter distributed throughout the disk. Furthermore, most of the mass of the luminous material (the stuff we can see; stars and interstellar gas and dust) is inside of the core. So you would expect that the rotation curves of spiral galaxies (outside their cores) should appear mostly Keplerian. This is not the case. Rather than falling off as 1/sqrt(r), the rotational velocity does not drop at all; it is "flat" all the way to the visible edge.

What does this imply? It implies that the assumption that most of the mass is in the core is wrong. The only way that the orbital velocity can remain constant is if the amount of mass within successive radii is increasing; meaning that the mass of the Galaxy is not distributed the way it appears to be (from the distribution of the luminous stars, dust, and gas).

This implies that there is some distribution of matter in the galaxy that we cannot see -- the so-called dark matter. In fact, given the amount of mass required to keep the galaxy rotating at it's high speed without flying apart, at least 75% of the mass of the galaxy must be dark matter.


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[This message has been edited by Borodog (edited 11-19-2002 02:51 PM).]

[Gomez]

How would we actually go about mining the asteroids?

[Borodog]

firemeboy: For years people thought we were at the center of the universe. Then we decided we're not. Now it looks like everything is expanding away from us at the same speed. Can you clarify this without saying 'the curve of space-time'?

Yes. First, everthing is not moving away from us at the same speed; the rate of recession of distant galaxies is proportional to their distance.

As far as explaining why we are not at the center, even though all (distant) galaxies are receding from us, in every direction, think about this:

There are many cities on the surface of the Earth. Which one is in "the middle" ? Clearly, none of them. Every point on the surface of the Earth is just as good as any other; from any city you can recieve letters from every direction, from every other city. Let's say that letters travel at a specific rate, such that a letter can get to a city all the way on the far side of the Earth in a week.

Now, imagine that the Earth is a balloon, and that that balloon is being inflated; it's expanding. The cities themselves are not expanding, they're made of bricks and mortar and are bound together by electromagnetic forces. But the ground they sit upon is expanding outward as the Earth gets larger. Now, no city moves relative to it's own piece of balloon , but the distance between all the cities gets larger. Each city looks at the postmarks on the letters they receive from distant cities, and they see that all of the cities are receding from them! And their rate of recession grows the farther away the particular city.

For example, imagine two cities, one a quarter way around the world, the other halfway around the world from a reference city. Let the Earth double in radius in time t. Since arc length is proportional to radius, the distance to each of these cities from the reference city must double. But that means that the city on the other side of the world must go twice as far as the city a quarter around, in the same time, and must hence be moving twice as fast.

The Universe behaves exactly analogously, only it's three dimensional and curved through a fourth, rather that the two dimensional surface of the Earth, which is curved through a third.





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[Borodog]

Gomez: How would we actually go about mining the asteroids?

I ain't no stinkin' engineer, buddy.


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[Robert Wheeler Willson]

What are your qualifications for answering these questions young man?

[Mikko]

Why should we care about what happens so far away that we can never reach it?

[firemeboy]

OK, here is one that even Hawking won't answer out right (though he certainly is leaning one way). Is time travel possible?

By the way, this book may bore Borodog, but it's a fun read for us lay people; The physics of Star Trek. It looks at all the stuf that happens in the Star Trek universe, and then explains what it would take to acutally make it work. It's a good introduction to physics...

[VinnyQ]

What was the name of the galaxy that the Milky Way is suppose to be merging with in a couple of billion of years? I saw it in a show somewhere but forgot the name.

[Quailman]

Yo, Viiny! It's sooner than you think!

[Borodog]

Bob Wilson: What are your qualifications for answering these questions young man?

I have a BS in physics, an MS in astrophysics, and I'm currently finishing my Ph.D. in astrophysics.

Mikko: Why should we care about what happens so far away that we can never reach it?

You shouldn't, unless you do. If you don't, then just stick to playing mafia and comparing post counts.

firemeboy: Is time travel possible?

I don't know. I tend to hope, for aesthetic reasons, that it is, but for practical reasons I think that it probably is not (at least not in any useful sense, like moving macroscopic collections of atoms, or even information bearing signals, backward in time). Time travel is my favorite topic for fiction writing, but the universe doesn't give a hoot about that.


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[Samadhi]

Yes, that's what I was speaking of. I'm a little rusty on my whatchamacallsits and whoosits. But aren't there other theories other than dark matter?
And what is this dark matter anyway that it only reactes gravitationally with other matter?

[Borodog]

VinnyQ: What was the name of the galaxy that the Milky Way is suppose to be merging with in a couple of billion of years? I saw it in a show somewhere but forgot the name.

The Milky Way may well "eat" many of its 12 or so satellite galaxies (small dwarf irregular galaxies such as the Small and Large Magellanic Clouds), but I assume you mean the collision with the Andromeda Galaxy, set to occur in about 3 billion years. The collission itself will last several hundred million years. See here for more information.



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[Borodog]

Quailman: Yeah, that's one of the small ones I mentioned.


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[Borodog]

Samadhi: But aren't there other theories other than dark matter?

And what is this dark matter anyway that it only reactes gravitationally with other matter?

No, there are no other viable theories than dark matter. The matter has to be there, and it's not shining, therefore, it's "dark matter."

As for what it is, there are a couple of theories: WIMPs and MACHOs. WIMPs are theorized as Weakly Interacting Massive Particles; they basically don't do anything but provide gravity. MACHOs would be MAssive Compact Halo Objects. These are theorized to be a huge number of small sub-stellar sized objects that do not shine and are hence (generally) undetectable. There is some support for the MACHO theory from observations of microlensing events; a massive compact object that passes exactly between us and a distant star will gravitationally disrupt the star's image. But this is expected to be incredibly rare, even if the galaxy is full of MACHOs.


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[Mikko]

I don't play mafia or compare post counts. Let me rephrase the question. Is there any advantage, apart from filling our curiosity, in understanding things that happen lightyears away?

Is it possible that the universe will reach a stable size? (That omega thing being exactly 1)

[Borodog]

Is there any advantage, apart from filling our curiosity, in understanding things that happen lightyears away?

Yes and no. Will it build a better microwave oven? Probably not. However, there are events that could take place many lightyears away that could wipe out life on Earth. But could you stop those events, even if you understood them and saw them coming? No.

But someday, I firmly believe, human beings will be in a position to investigate these things first hand, and the science we do today will be the foundation upon which those expeditions work against.

Is it possible that the universe will reach a stable size? (That omega thing being exactly 1)

No. If Omega=1, that means that the expansion of the Universe would halt at t=infinity. By definition, you will never get to t=infinity, therefore the outward expansion velocity will never be zero. If it ever were to drop to zero at some finite time, the Universe would then begin to collapse.

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[Hitchhiker]

Can you explain the "red shift?"

Are there particles smaller than quarks?

Are tachyons real?

[Samadhi]

But why all this stuff "billions of light years away" when the universe is just 10,000 years old?
And why all this talk about galaxies running into us in a few hundred million years when Armageddon is scheduled this century?

[Stubby]

Are you the person I talk to if I want to have a galaxy named after me?

[Samadhi]

Oh and here's a good one (seriously). What would you like to do with your PhD?
(Dr. Borodog. Hmmm.)

[Zarriar]

Is it true that the speed of light is decreasing over time? If true, what are the implications of this?

[Borodog]

Can you explain the "red shift?"

"Red shift" refers to the shift to longer wavelength of electromagnetic radiation. Visible light would be shifted towards the red end of the spectrum. There is also blue shift. The term is actually generic, in that it need not refer to visible light at all. gamma rays can be red shifted to X-rays, microwaves, ultraviolet, visible, infrared, or radio. Any of those wavelengths can be red shifted down to a longer wavelength (lower energy), or blue shifted up to a shorter wavelength (higher energy).

There are three sources of red shift: Doppler red shift, gravitational red shift, and cosmological red shift.

Doppler red shift occurs when an object emitting electromagnetic radiation is moving away from you. If the object were sitting still relative to an observer, the distance between subsequent wave crests is the same for both observer and emitter. If however, the emitter is moving away, the distance between subsequent crests will be larger for the observer, hence the wave has a longer wavelength; it has been Doppler red shifted. The reverse (an object moving towards the observer) will produce a Doppler blue shift. You can hear the same effect as trains or cars approach and pass you; the note of their engines or horns will change; it will be higher on the approach and lower as it departs. Doppler red and blue shifts have been observed in many astrophysical situations, the rotation curves of galaxies, the motions of nearby galaxies, spectroscopic binaries, accretion disk spectra, etc, etc, etc.

Gravitational red shift occurs near massive objects that warp spacetime. Time runs slower near a massive object; hence to a distant observer, successive wave crests will be emitted more slowly, leading again to a longer wavelength, and a red shift. The object does not need to be moving at all; just the proximity of a massive object will create this effect. This effect has been measured using atomic clocks to detect the time dilation caused by the mass of the Earth (surprise surprise, it is small).

Cosmological red shift occurs because the universe is expanding. Note that this is NOT a Doppler red shift caused by the recession of all the galaxies! If that were the case, we could not observe recessional velocities greater than the speed of light (we do). Rather, this red shift is caused by the expansion of space itself. Recall the expansion of the balloon; as successive letters (wave crests) traverse the Universe, the distance between them increases; this again gives us a longer wavelength and a red shift. This cosmological red shift has been measured many hundreds of thousands of times using distant galaxies.

Are there particles smaller than quarks?

Not that I know of, but the energies that would be required to probe the internal structure of quarks are very large. Even if there were, it's unlikely we'll be able to detect them soon. However some make a good abstract argument for why there might be particles smaller than quarks.

Are tachyons real?

Almost certainly no. Tachyons are a valid solution to the mathematical equations that describe the propagation of particles/waves, but there is no evidence that the solution actually describes something that exists (tachyons are the so-called "advanced wave" solutions; our world is composed of the "retarded wave" solutions, as un-pc as that sounds). Tachyons would travel faster than light, have imaginary mass, slow down when they gain energy, violate causation, and most importantly, have observable implications (Cerenkov radiation). They have never been observed, and are near uniformly dismissed. However, it is a conundrum that there is nothing in the mathematical description that prevents their existence.

What would you like to do with your PhD?

Mostly, I want to be able to put astrophysicist on the jackets of my science fiction novels. After that I want to teach and do research. I absolutely love to teach, and the research that I do is supercool, IMHO.

Is it true that the speed of light is decreasing over time? If true, what are the implications of this?

No, this is not true. This is a crock of crap that Young Earth Creationists like to trot out to avoid the fact that all of modern astronomy, astrophysics, and cosmology totally debunk their pet theory. It is true that both the accuracy and precission with which people measured the speed of light has increased. All measurements however, going back over 400 years, agree within their respective margins of error. The implications are that Creationists will go to any loony length to avoid having to give up their preconceived notions.


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[Bicho the Inhaler]

If the speed of light were to change, what would it change relative to?

[Werebear]

How much lint is in my belly button right now?

[PuzzleProdigy]

*gasp*

Borodog called creationism a theory!
He almost has a PhD, it must be true!

*goes to tell the world*

[Samadhi]

What's up with those stars off the main sequence?

[Lucky Wizard]

Hope I'm allowed to answer Samadhi's question.

First, a bit of background for readers who are unaware: As you might know, when a star forms, it has a temperature and a brightness. At the beginning of a star's life, the temperature can usually be predicted from the brightness, and vice versa. This correlation is the "main sequence" Samadhi referred to. Cool stars are generally dim, and hot stars are generally bright.

Anyway, to Samadhi's question: Stars start out on the main sequence. But they don't stay there. At a certain time (which takes a loooong time), which depends on the star's mass, the star undergoes a change, which takes it off the main sequence. When a star is more massive than a certain mass, it will eventually expand. The sun, for instance, will eventually expand into a red giant star. Now, red stars are relatively cool stars, so a red main sequence star would be dim, right? But red giants aren't main sequence. They're far brighter than a red main sequence star (which, by the way, is a dim red dwarf like Barnard's star).

So the basic answer to your question is this: Many stars are massive enough that at some point in time, they'll undergo physical changes that make them cooler yet brighter, which at first appears to be a paradox.

[Zarriar]

Borodog, I'm very sorry I just assumed you had heard about this:

http://www.cbsnews.com/stories/2002/08/07/tech/main517850.shtml

I wasn't referring to Creationist Theory.

[Borodog]

Of course Young Earth Creationism is a theory. It's even a scientific theory. It makes fundamental predictions about the way the world should look that you can go out and test; for instance the hydraulic sorting of fossils one would expect from a Biblical, world drowning flood in 2000 BC. It fails all of these tests, which is how we can definitely say that it is false, with 100% certainty.

Lucky Wizard,

The explanation you gave Samadhi is rather empirical in nature, and while mostly accurate, I fear it's doesn't answer the fundamental conceptual question.

There certainly is a temperature luminosity relationship among most stars over the majority of their lifetimes. This comes about in the following manor.

A star is a ball of gas that has collapsed from a cloud under its own gravity. Gravity compresses this ball of gas, raising the pressure in the core, until the point where hydrogen fusion begins. Once this happens, the temperature and (pressure) in the core go up until the pressure in the core balances the gravitational force, and the star stops collapsing. It enters a condition called hydrostatic equilibrium. Now, the more massive the star, the larger the gravitational force, the large the temperature and pressure required in the core, the higher the rate of nuclear fusion. So in essence, more massive stars burn hotter and faster.

However, temperature is only one of two components that determine a star's luminosity, the other being it's size. A large star and a small star, both having the same surface temperature, will have luminosities proportional to their surface areas; thus the larger star will be brighter. Luckily for us, both of these quantities (temperature and radius) are determined solely by the star's mass over most of it's lifetime, giving us a (relatively) simply mass-luminosity relationship:

log(L/L_sun)=3.8*log(M/M_sun)+0.8

Now, once a star has exhausted it's initial nuclear fuel (hydrogen), all bets are off. Depending on the stars mass, it can begin to fuse higher and higher elements (up to a cutoff determined by the mass), and new equilibria are struck between temperature and radius, hence giving different luminosities.

The Main Sequence can be seen in several different graphs, but the simplest one is perhaps the temperature-luminosity diagram. All stars on the main sequence lie on a curve from hot & bright to cool and dim.

When stars burn up their fuel, they leave the main sequence. By the way, you can date the ages of clusters by plotting a relative of this type of diagram and looking for the "turn-off" from the main sequence; the position of the turn-off tells you how old the cluster is, because you know the oldest stars are the ones that have just turned, and you know their main sequence lifetimes because you know how massive their are, and hence how fast they burn, and how long they live on the main sequence.

Anyway, upon leaving the main sequence, stars generally get cooler (but massive stars do not, really massive stars do briefly), but also bloat up to be subgiants, then giants, then giants, supergiants, and even hypergiants, so they get cooler but much brighter. So stars, upon leaving the Main Sequence, will generally slide around the various giant branches for ever shorter periods of time (until they're reached the limit of higher atomic mass nuclear fuel that their cores can burn), and end up in one of two deaths:

If they're relatively low mass (less than about 8 solar masses), they'll burn away so much mass that the burning of higher elements at the core basically gently blows the outer portions of the star off in a planetary nebula, leaving the burned out core to finally collapse to a white dwarf, where gravity is finally balanced by electron degeneracy pressure. These stars are generally balls of carbon/oxygen (maybe with some neon) less than 1.4 solar masses. They are extremely hot, but very small, and hense are very dim. They radiate away their heat, cooling. Eventually they will become cold black dwarves.

If they're more than around eight solar masses, they will undergo core collapse after they fuse all their fuel up to iron (iron fusion is an endothermic, rather exothermic reaction; hence you can't get energy out to support the core). These stars are so massive that electron degeneracy pressure cannot support them, and they continue to collapse. They collapse to the point that neutron degeneracy pressure kicks in, the core suddenly becomes stiff, and the infalling star bounces, forming an outgoing shockwave. This shock is strengthened by neutrinos that exit the core as the iron core is converted entirely into neutrons, to the point where the shockwave blows most of the star off in a massive explosion: a core collapse supernova. If the progenitor was less than around 20 solar masses, a neutron star will result, with a mass from 1.4 to 3 solar masses. If the progenitor star was more massive, after the initial stiffening that bounces the outer layers off in a supernova, the core will continue to collapse into a black hole.

So in short, the main sequence runs from bright-hot to dim cool. Star that turn off the main sequence generally get brighter and cooler as they enter various giant phases in their golden years. Low mass stars end their lives by gently puffing off their outer envelopes ending up as small, hot, but dim stars that eventually will cool and fade out. High mass stars die in cataclysmic explosions that leave neutron stars or black holes.



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[Borodog]

Ah, no, I hadn't seen that. I'll reserve comment until the preponderance of the evidence shifts to "the speed of light is changing".

It strikes me as highly unlikely, but then again, inflation strikes me as highly unlikely.


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[Zarriar]

Do galaxies combine to form larger structures? If so what do they look like?

[Samadhi]

Could you give an idea of how quickly the shock wave onsets, propagates etc?

[Edit: english]

[This message has been edited by Samadhi (edited 11-20-2002 03:21 AM).]