Starship Asterisk*

Ann, I'm still trying to understand how you are standing in for me. I don't even know what you mean by open versus closed universe. For the record, I would personally prefer the fate of the universe to be a Big Crunch, not a Big Chill, but I try not to let it bother me.

I will note, however, that the something may expand at an accelerating, constant, or decelerating rate. In each case, it is still expanding. It is only if the deceleration continues for long enough to stop and reverse the expansion, that it will begin to contract.

I was wondering what the movement is of globular clusters. Being so compact is there a gravitational balance throughout most of the structure? Is there an axis of rotation of the entire cluster? There must be a pretty precise balance to keep it all from collapsing in on itself. Do the individual stars rotate?

Paul Douglas wrote:I was wondering what the movement is of globular clusters. Being so compact is there a gravitational balance throughout most of the structure? Is there an axis of rotation of the entire cluster? There must be a pretty precise balance to keep it all from collapsing in on itself. Do the individual stars rotate?

The individual stars all orbit the center of mass in globular clusters, which might contain an intermediate massed black hole. All stars should have at least some rotation, which just means spinning on its own axis.

Ann wrote:

Anthony Barreiro wrote:
If there were cosmologists more than five billion years ago, they would have observed that the expansion of the universe was decelerating, and would have reasonably predicted an eventual "big crunch."

Sorry about standing in for Nitpicker here, but since I am so relieved that we are apparently not headed for a Big Crunch, I have to defend the open universe. (I was about to write "the open university"!)

If the universe was already decelerating, then we would undoubtedly be headed for a Big Crunch. But the universe has never been decelerating. It was the acceleration of the universe that was slowing down for a while, not the universe itself. When the two teams headed by Saul Perlmutter and Brian Schmidt and Adam Riess were chasing distant supernovas, their aim was to find out how much the acceleration of the universe was decelerating. By finding out how much the acceleration of the universe was being slowed down by the effects of gravity, the teams wanted to find out if the universe was closed or not. They took it for granted that the acceleration of the universe was slowing down, but they didn't take it for granted that the universe was closed. It depended on how much the universe was slowing down. Of course, what they found was that the acceleration of the universe was speeding up.

Now imagine that there were astronomers and cosmologists more than five billion years ago. If they measured the acceleration of the universe at that time, they would indeed find that the acceleration was slowing down. But they would also find that the slowing down of the acceleration was not enough to ever "turn the universe around" and eventually make it collapse. They, too, would have concluded that they lived in an open universe.

Ann

Ann, my very superficial understanding, from Pasachoff and Filippenko's The Cosmos: Astronomy in the New Millennium, is that the universe has always been expanding, but the rate of acceleration has changed. To quote page 502: "the data show that for roughly the first 9 billion years of its existence, the Universe was decelerating. Then, 4 or 5 billion years ago, the expansion rate began to accelerate." Again, I think about driving my car on the freeway. If I press my foot down on the gas pedal I go faster. If I take my foot off the gas pedal I go slower. But in either case I'm still moving forward.

By the way, I am firmly agnostic about the geometry and fate of the universe.

Chris Peterson wrote:

Anthony Barreiro wrote:

Chris Peterson wrote:This is the source of much confusion. High redshift galaxies are not old. They're young.

In the sense that they were young when they emitted the light we're seeing today.

But we always look at things as having the age where we observe them. Thus, if we see a supernova tonight that's a million light years away, we consider the supernova brand new, not a million years old. The same is true for redshifted galaxies. What makes them interesting is they provide a window into how galaxies formed, because we can see them very young.

But the light itself is very old.

Whatever that means. Light is just energy, and all the energy in the Universe appeared at the instant of the Big Bang.

Yes, but the energy has been going through interesting changes in the mean time. One way to think about it is that the light from a high red shift galaxy is carrying information about something that happened a long time ago.

Given that we are considering phenomena so far outside a normal human frame of reference, the presumption that there is only one correct way to think about this stuff seems rather small minded.

Anthony Barreiro wrote:Given that we are considering phenomena so far outside a normal human frame of reference, the presumption that there is only one correct way to think about this stuff seems rather small minded.

I don't think I said anything to suggest otherwise.

I was only pointing out that there is a good deal of confusion about what it means to look at high redshift objects. It is common to think that the further away something is, the older it is. That is incorrect. The farther away something is, the further in the past we see it. For cosmological distances, that means the farther away something is, the younger we see it. I'm hard pressed to think of another "correct" way to see this particular point.

Chris Peterson wrote:

Anthony Barreiro wrote:Given that we are considering phenomena so far outside a normal human frame of reference, the presumption that there is only one correct way to think about this stuff seems rather small minded.

I don't think I said anything to suggest otherwise.

Chris Peterson wrote:

Anthony Barreiro wrote:But the light itself is very old.

Whatever that means. ...

Q.E.D.

Anthony Barreiro wrote:

Chris Peterson wrote:

Anthony Barreiro wrote:Given that we are considering phenomena so far outside a normal human frame of reference, the presumption that there is only one correct way to think about this stuff seems rather small minded.

I don't think I said anything to suggest otherwise.

Chris Peterson wrote:

Anthony Barreiro wrote:But the light itself is very old.

Whatever that means. ...

Q.E.D.

Sorry, I don't follow. A comment implying that further clarification is required does not support your assertion. Non quod erat demonstrandum sed aliud.

Chris Peterson wrote:Sorry, I don't follow. A comment implying that further clarification is required does not support your assertion. Non quod erat demonstrandum sed aliud.

Chris, does this really read as a straightforward request for clarification?

Chris Peterson wrote:Whatever that means. Light is just energy, and all the energy in the Universe appeared at the instant of the Big Bang.

If you require clarification, please say so directly.

Anthony Barreiro wrote:Chris, does this really read as a straightforward request for clarification?

Chris Peterson wrote:Whatever that means. Light is just energy, and all the energy in the Universe appeared at the instant of the Big Bang.

Yes. I think a comment "whatever that means" in response to "the light itself is very old" is pretty clearly saying that I don't know what you're trying to say. (And really, I still don't, outside your clarification that it had been a long time in flight.)

The remainder of the sentence is just a simple statement of fact, of course.

Anthony Barreiro wrote:By the way, I am firmly agnostic about the geometry and fate of the universe.

I was just about to agree with you on this. But then I realised I was happier to accept the current consensus view of scientists in this field, which I understand to be an open universe and a Big Chill. Note also that an open universe does not necessarily imply a Big Chill and vice-versa.

If the consensus view ever changes, then so will mine. Some might call this my faith in science (as I do), but it does have a rational basis to it.

Nitpicker wrote:

Anthony Barreiro wrote:By the way, I am firmly agnostic about the geometry and fate of the universe.

I was just about to agree with you on this. But then I realised I was happier to accept the current consensus view of scientists in this field, which I understand to be an open universe and a Big Chill. Note also that an open universe does not necessarily imply a Big Chill and vice-versa.

If the consensus view ever changes, then so will mine. Some might call this my faith in science (as I do), but it does have a rational basis to it.

Your view is completely rational and scientific. Being "agnostic" is the faith-based position for an educated non-specialist.

Always go with the consensus view, particularly when the consensus is a significant one.

Nitpicker wrote:

Anthony Barreiro wrote:By the way, I am firmly agnostic about the geometry and fate of the universe.

I was just about to agree with you on this. But then I realised I was happier to accept the current consensus view of scientists in this field, which I understand to be an open universe and a Big Chill. Note also that an open universe does not necessarily imply a Big Chill and vice-versa.

If the consensus view ever changes, then so will mine. Some might call this my faith in science (as I do), but it does have a rational basis to it.

I'm happier to have a positive opinion that neither the Big Crunch nor the Big Chill will occur and that our universe will always remain open. My basis for believing this would be considered irrational to many, but not to me.

Ann wrote:

Anthony Barreiro wrote:
If there were cosmologists more than five billion years ago, they would have observed that the expansion of the universe was decelerating, and would have reasonably predicted an eventual "big crunch."

Sorry about standing in for Nitpicker here, but since I am so relieved that we are apparently not headed for a Big Crunch, I have to defend the open universe. (I was about to write "the open university"!)

If the universe was braking hard enough, then we would undoubtedly be headed for a Big Crunch. But the universe has never been slowing down that sharply. The expansion of the universe that was slowing down for a while, but the universe kept expanding. When the two teams headed by Saul Perlmutter and Brian Schmidt and Adam Riess were chasing distant supernovas, their aim was to find out by how much gravity was slowing down the expansion of the universe. By finding out how much the expansion of the universe was being slowed down by the effects of gravity, the teams wanted to find out if the universe was closed or not. They took it for granted that the expansion of the universe was slowing down, but they didn't take it for granted that the universe was closed. It depended on how much the universe was slowing down. Of course, what they found was that the expansion of the universe was speeding up.

Now imagine that there were astronomers and cosmologists more than five billion years ago. If they measured the expansion of the universe at that time, they would indeed find that the expansion was slowing down. But they would also find that the slowing down of the expansion was not enough to ever "turn the universe around" and eventually make it collapse. They, too, would have concluded that they lived in an open universe.

Ann

Ooops! I meant to edit my post. Somehow, instead, I cloned it. Could an administrator please put the above, edited post of mine where my post way down on page two is?

Ann

EDIT: I just received a pm from Geckzilla, where she advised me to keep both versions of my post. I have made substantial changes to it, and Anthony Barreiro has replied to the first version of my post. His reply might be hard to understand if my first post is removed. Therefore I'm going to keep both. Please note that the post above this one is the version that I much prefer.

Ann wrote:Now imagine that there were astronomers and cosmologists more than five billion years ago. If they measured the expansion of the universe at that time, they would indeed find that the expansion was slowing down. But they would also find that the slowing down of the expansion was not enough to ever "turn the universe around" and eventually make it collapse. They, too, would have concluded that they lived in an open universe.

I don't think that is true (or at least, far from certain). When gravity was the dominant force at cosmological distances, the decreasing rate of expansion may well have been a strong indicator that the Universe was closed. You need more than just knowledge of the rate of expansion at different times, you also need an accurate value for the total energy (mass) of the Universe.

Chris Peterson wrote:

Ann wrote:Now imagine that there were astronomers and cosmologists more than five billion years ago. If they measured the expansion of the universe at that time, they would indeed find that the expansion was slowing down. But they would also find that the slowing down of the expansion was not enough to ever "turn the universe around" and eventually make it collapse. They, too, would have concluded that they lived in an open universe.

I don't think that is true (or at least, far from certain). When gravity was the dominant force at cosmological distances, the decreasing rate of expansion may well have been a strong indicator that the Universe was closed. You need more than just knowledge of the rate of expansion at different times, you also need an accurate value for the total energy (mass) of the Universe.

So let's assume that those astronomers who lived in a decelerating universe were in fact able to measure the total energy of the universe, for example by studying the cosmic microwave background. In your opinion, Chris, would they have determined that they lived in an open or a closed universe?

Ann

Ann wrote:So let's assume that those astronomers who lived in a decelerating universe were in fact able to measure the total energy of the universe, for example by studying the cosmic microwave background. In your opinion, Chris, would they have determined that they lived in an open or a closed universe?

That's a good question. In fact, it's precisely where human astronomers were just a decade ago. The problem with answering this is that 68% of the energy budget of the Universe is dark energy. So to accurately measure the mass of the Universe requires being aware of dark energy, and that means understanding its role in affecting universal expansion. Astronomers from 5 billion years ago lived in a gravity dominated universe. We learned about dark energy by observation; they probably would not have been able to do so. So if they understood dark energy, they would have developed that knowledge in some different way.

Thanks all for the replies to my post on April 26. For some reason I only received an email a short time ago (dated 29 Apr 2014 22:24) informing me that there had been a reply yet the first response was on April 26th.

Has anyone got a link to an image of what the night sky would look like from the centre of a globular cluster like this. I have often wondered.

AQ

Anthraquinone wrote:Has anyone got a link to an image of what the night sky would look like from the centre of a globular cluster like this. I have often wondered.

AQ

Attempts have been made. I haven't seen one that I thought was great, but Astrobob's description of thousands of stars as bright as Venus and Jupiter seems like it wouldn't be too hard to accurately portray but all of the illustrations I have seen have taken too many liberties and made little or no effort to look accurate. There was once an APOD on the matter but I have to warn you... it's digitally manipulated birds in the sky. Personally I found that image to be fairly unsatisfying.