New to this site (very first post), and have a question regarding the accelerating expansion of the universe, and the reason scientists think this is so. Maybe someone could help me understand this.
The question occurred to me last night while watching the "Universe" series about dark matter and dark energy.
I can understand the concept of dark matter and the gravitional effects it exerts on the universe, and also understand that dark matter is supposedly exerting some kind of repulsive force, and hence accelerating the expansion rate.
I can understand how the distance to another deep space galaxy can be established using the super nova light brilliance method, and I understand how red/blue shift will deteremine whether an object is moving toward us, or away.
What I don't understand is this:
The distances are so vast that we are actually looking far back in time, sometimes billions of years. If it is determined that a far off galaxy is increasing its rate of acceleration away from us, by definition is has only been determined that this acceleration was happening so many million, or billions, of years ago.
How do we know that this is still happening today?
Isn't it conceivable that the expansion rate has slowed, stopped, or even reversed itself, and the universe could actually be contracting at this time?
How would we know, if we can't see present day conditions across the vastness of the universe?
Thanks in advance to all who might be able to help answer this.
QUOTE (doc737ng+Feb 6 2008, 06:53 PM)
New to this site (very first post), and have a question regarding the accelerating expansion of the universe, and the reason scientists think this is so. Maybe someone could help me understand this.
The question occurred to me last night while watching the "Universe" series about dark matter and dark energy.
I can understand the concept of dark matter and the gravitional effects it exerts on the universe, and also understand that dark matter is supposedly exerting some kind of repulsive force, and hence accelerating the expansion rate.
The expansion is not related to dark matter except that it associated with an energy source that has been given the unfortunate name of "dark energy".
Roughly, what we know is that the far off galaxy does not look like it's going away from us as fast as the ones near to us. (This indicates that the ones near us are going faster than galactic expansion in the past.)
We assume that, because the universe looks pretty much the same wherever we look, that it actually is the same no matter where we look. This limits the laws of physics that we can use to describe the universe. That is, we assume that the same laws of physics apply to the same kinds of stuff and that the same kinds of stuff is what's out there.
Roughly, what we know is that the far off galaxy does not look like it's going away from us as fast as the ones near to us. (This indicates that the ones near us are going faster than galactic expansion in the past.)
We assume that, because the universe looks pretty much the same wherever we look, that it actually is the same no matter where we look. This limits the laws of physics that we can use to describe the universe. That is, we assume that the same laws of physics apply to the same kinds of stuff and that the same kinds of stuff is what's out there.
Isn't it conceivable that the expansion rate has slowed, stopped, or even reversed itself, and the universe could actually be contracting at this time?
It is always possible that there is some new force or forces that act in ways totally different from ones that we have observed. However, given the observed homogeneity of the universe, we make certain inferences to the bahaviour of the spacetime of the universe given the rules laid down by General Relativity.
Given the available observations, we've seen that the universal expansion slowed down initially and then began to speed up a couple of billion years ago.
The question occurred to me last night while watching the "Universe" series about dark matter and dark energy.
I can understand the concept of dark matter and the gravitional effects it exerts on the universe, and also understand that dark matter is supposedly exerting some kind of repulsive force, and hence accelerating the expansion rate.
The expansion is not related to dark matter except that it associated with an energy source that has been given the unfortunate name of "dark energy".
QUOTE
I can understand how the distance to another deep space galaxy can be established using the super nova light brilliance method, and I understand how red/blue shift will deteremine whether an object is moving toward us, or away.
What I don't understand is this:
The distances are so vast that we are actually looking far back in time, sometimes billions of years. If it is determined that a far off galaxy is increasing its rate of acceleration away from us, by definition is has only been determined that this acceleration was happening so many million, or billions, of years ago.
How do we know that this is still happening today?
What I don't understand is this:
The distances are so vast that we are actually looking far back in time, sometimes billions of years. If it is determined that a far off galaxy is increasing its rate of acceleration away from us, by definition is has only been determined that this acceleration was happening so many million, or billions, of years ago.
How do we know that this is still happening today?
Roughly, what we know is that the far off galaxy does not look like it's going away from us as fast as the ones near to us. (This indicates that the ones near us are going faster than galactic expansion in the past.)
We assume that, because the universe looks pretty much the same wherever we look, that it actually is the same no matter where we look. This limits the laws of physics that we can use to describe the universe. That is, we assume that the same laws of physics apply to the same kinds of stuff and that the same kinds of stuff is what's out there.
QUOTE (->
| QUOTE |
| I can understand how the distance to another deep space galaxy can be established using the super nova light brilliance method, and I understand how red/blue shift will deteremine whether an object is moving toward us, or away. What I don't understand is this: The distances are so vast that we are actually looking far back in time, sometimes billions of years. If it is determined that a far off galaxy is increasing its rate of acceleration away from us, by definition is has only been determined that this acceleration was happening so many million, or billions, of years ago. How do we know that this is still happening today? |
Roughly, what we know is that the far off galaxy does not look like it's going away from us as fast as the ones near to us. (This indicates that the ones near us are going faster than galactic expansion in the past.)
We assume that, because the universe looks pretty much the same wherever we look, that it actually is the same no matter where we look. This limits the laws of physics that we can use to describe the universe. That is, we assume that the same laws of physics apply to the same kinds of stuff and that the same kinds of stuff is what's out there.
Isn't it conceivable that the expansion rate has slowed, stopped, or even reversed itself, and the universe could actually be contracting at this time?
It is always possible that there is some new force or forces that act in ways totally different from ones that we have observed. However, given the observed homogeneity of the universe, we make certain inferences to the bahaviour of the spacetime of the universe given the rules laid down by General Relativity.
Given the available observations, we've seen that the universal expansion slowed down initially and then began to speed up a couple of billion years ago.
PhysBang, thanks for the reply.
So what you are saying is that the nearer galaxies, which are being viewed in a more recent time frame, appear to be accelerating at a faster rate than the far off galaxies, which are seen in a much earlier time? And since the nearer galaxies are more indictitive of what's actually happening at the present time, then we must presume, all things being equal, that the far off galaxies are accelerating as well? We don't actually have evidence, as such, but rather infer this through the observations of the nearer galaxies?
The closest galaxy is 2 million light years away from us. We are looking back in time 2 million years. So how do we know this is still happening, present time? The best observation we have is now 2 million years old.
Bear in mind, I'm no science guru; just an ordinary guy with an interest in how, and why, things are as they are.
I don't wish to appear dense, but this is just really difficult to wrap my head around.
Thanks
So what you are saying is that the nearer galaxies, which are being viewed in a more recent time frame, appear to be accelerating at a faster rate than the far off galaxies, which are seen in a much earlier time? And since the nearer galaxies are more indictitive of what's actually happening at the present time, then we must presume, all things being equal, that the far off galaxies are accelerating as well? We don't actually have evidence, as such, but rather infer this through the observations of the nearer galaxies?
The closest galaxy is 2 million light years away from us. We are looking back in time 2 million years. So how do we know this is still happening, present time? The best observation we have is now 2 million years old.
Bear in mind, I'm no science guru; just an ordinary guy with an interest in how, and why, things are as they are.
I don't wish to appear dense, but this is just really difficult to wrap my head around.
Thanks
Hi!
The Small Magellanic Cloud is only at 200kLY, less than 2MLY.
At such small distances, we see only random galactic speeds. We need greater distances to see expansion speeds big enough to overshadow the disorder and get a global picture of expansion.
Distances of galaxies is estimated by one kind of supernova thought to bring a repeatable absolute brightness (magnitude); By measuring the apparent brightness, astronomers pretend to deduce the distance of the supernova and its parent galaxy.
Drawing a diagram with distance (supernova) and speed (red shift) tells that expansion should be accelerating, so cosmologists wanting to keep the standard big bang [that is, almost all of them] introduce a dark energy.
Slight changes to the mass attraction formula would also work. Not more, not less ad hoc than dark energy. Big-scale fluctuations in the population density of the Universe is said to favour the dark enery.
However, other people recently suggested that these supernovae actually are of various types with various absolute brightnesses. This could alter the situation.
Or could it be that supernovae exploded differently in a young universe?
Once again, a theory depends on a model... Uncomfortable. And since we can't properly make cosmology in a lab, cosmology will remain a half-science.
The Small Magellanic Cloud is only at 200kLY, less than 2MLY.
At such small distances, we see only random galactic speeds. We need greater distances to see expansion speeds big enough to overshadow the disorder and get a global picture of expansion.
Distances of galaxies is estimated by one kind of supernova thought to bring a repeatable absolute brightness (magnitude); By measuring the apparent brightness, astronomers pretend to deduce the distance of the supernova and its parent galaxy.
Drawing a diagram with distance (supernova) and speed (red shift) tells that expansion should be accelerating, so cosmologists wanting to keep the standard big bang [that is, almost all of them] introduce a dark energy.
Slight changes to the mass attraction formula would also work. Not more, not less ad hoc than dark energy. Big-scale fluctuations in the population density of the Universe is said to favour the dark enery.
However, other people recently suggested that these supernovae actually are of various types with various absolute brightnesses. This could alter the situation.
Or could it be that supernovae exploded differently in a young universe?
Once again, a theory depends on a model... Uncomfortable. And since we can't properly make cosmology in a lab, cosmology will remain a half-science.
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