questions about dark matter

[Janus]

13 hours ago, mistermack said:

Thanks. That's a pretty stunning picture, even if it is the logical consequence of not interacting. 

Presumably the same can't happen with a black hole, or can it? Once inside the event horizon there is no path out so I'm guessing it would be trapped inside. 

Right. The thing with dark matter is that the initial trajectory would need to be such that it took it past the EH.  With visible matter it can have an initial trajectory that takes it outside the EH and back out into space again, but through collisions with other visible matter, it can shed energy and have its trajectory altered into a EH crossing one. (it is also conceivable that an close approach with something else could gravitationally alter a dark matter particle into a EH crossing path, but it could also alter a EH crossing path into one that misses.  With visible matter collisions, energy is given up via radiation leaving both parties with less KE than they started with.  This increases the statistical averages for visible matter in terms of falling inside the EH.

[boo]

Quote

Boo Said:  is dark matter nothing more than a wildcard thrown in to a mathematical equation to make the equation work?

 

On 9/18/2019 at 2:49 PM, Strange said:

I don't know what that means. Neptune was a "wildcard thrown in to a mathematical equation to make the equation work". The same could be said of electrons, photons, gravity, energy ...

meaning that it was invented to explain the expanding universe, rather than because we detected it or "saw" it or otherwise proved it to be there.  thats what i meant, with the question.

reviving an old thread here, but thought i would clarify what i meant there.

Edited October 15, 2019 by boo

[Strange]

21 minutes ago, boo said:

meaning that it was invented to explain the expanding universe, rather than because we detected it or "saw" it or otherwise proved it to be there.  thats what i meant, with the question.

Dark matter was originally proposed to explain the unexpected orbital speeds of galaxies in clusters. Then it was found that the same thing applied to the orbits of stars within galaxies.

Then it was found to be necessary to explain large structure formation. And signals in the CMB. And gravitational lensing.

So it has been detected in multiple ways. It just hasn't yet shown up in any attempts to detect the specific particles involved (if there are any).

Dark energy is relevant to the expansion of the universe; it is the simplest way of describing the accelerating expansion (not the expansion; that would happen anyway).

[Janus]

2 hours ago, boo said:

 

meaning that it was invented to explain the expanding universe, rather than because we detected it or "saw" it or otherwise proved it to be there.  thats what i meant, with the question.

reviving an old thread here, but thought i would clarify what i meant there.

As Strange as pointed out, Dark matter, is actually proposed to explain why the observable universe behaves as if it has more mass than we can see in the form of visible matter.

Dark energy is the term given to whatever is causing the expansion of the universe to accelerate ( you don't need dark matter for the universe to expand, only for that expansion rate to increase over time.)

Both contain the word "dark", but that is just due to laziness when it came to naming conventions. "dark" matter was chosen because the matter didn't interact with light.  "dark" energy was chosen, because "Hey, we already have "dark" matter", and not because there was any suspected relationship between the two.

[boo]

wow iv learned a lot in those two answers.  I didnt realise that dark matter and dark energy were two different things

incidentally, how do we know that the expansion is increasing? and how is it increasing exactly?

I always thought it was increasing at a steady rate,  but naturally that would mean the farther something is away, the faster it would move away simply because the space in between is expanding.

is that it? 

or am i to believe that things are expanding faster than they used to?   as in,  lets say you had two galaxies a certain distance from one another. would they move away from eachother at a faster rate now than they would have a billion years ago?  

 

[Strange]

1 minute ago, boo said:

wow iv learned a lot in those two answers.  I didnt realise that dark matter and dark energy were two different things

incidentally, how do we know that the expansion is increasing? and how is it increasing exactly?

I always thought it was increasing at a steady rate,  but naturally that would mean the farther something is away, the faster it would move away simply because the space in between is expanding.

is that it? 

or am i to believe that things are expanding faster than they used to?   as in,  lets say you had two galaxies a certain distance from one another. would they move away from eachother at a faster rate now than they would have a billion years ago?  

Both of the above!

Firstly, we know the rate of expansion mainly from measuring the red-shift of distant galaxies and comparing that to their distance.

It was noticed about 100 years ago, that there is a linear correlation between distance and red-shift. The Hubble-Lemaitre law: the red-shift of distant galaxies (equivalent to their recessional speed) is proportional to their distance. This increase in speed with distance is not "acceleration", it is just expansion.

(There is a lot behind that, which I don't really want to go into. For example, how do we know the distance: https://en.wikipedia.org/wiki/Cosmic_distance_ladder. And other ways of measuring the rate of expansion give slightly different answers, which may indicate new things to be discovered.)

Important thing to understand is that expansion is a scaling effect (so when people say things like "expanding at the speed of light" it is completely meaningless). As a result, the speed at which any two points separate is proportional to how far apart they are. This is just simple arithmetic, nothing to do with cosmology.

Consider a number of galaxies separated by the same distance (far enough apart that the expansion of space is significant and the same between all of them).

At time 0, they are 1 unit apart:
A.B.C.D.E.F

After some time they are 2 units apart:
A..B..C..D..E..F

After the same time again, they are 3 units apart:
A...B...C...D...E...F

And so on:
A....B....C....D....E....F

Now, if we look at the distance between B and C, for example, it increases by 1 at every time step. But the distance between B and D increases by 2 at every step. So the distance between B and D is increasing twice as fast as the distance between B and C; i.e. the speed of separation is twice as great.

Choose any pairs of galaxies and you will see that apparent the speed of separation is proportional to the distance between them. Take two objects far enough apart and the speed of separation will be greater than the sped of light. 

This is basically the cause of the Hubble-Lemaitre law.

Then in 2011 a team of scientists got the Nobel Prize for having discovered that the relationship is not completely linear. It seems that at some point (about 5 billion years ago?) the rate of expansion had started increasing. The easiest way to model this in the equations describing the expansion is to add a new energy term which is constant for a given volume of space. As space expands, this energy term increases. This is called "dark energy". No one knows if this is the correct explanation (ie if there really is a thing called dark energy) or if this is just a useful way of modelling it.

 

[Janus]

4 hours ago, boo said:

wow iv learned a lot in those two answers.  I didnt realise that dark matter and dark energy were two different things

incidentally, how do we know that the expansion is increasing? and how is it increasing exactly?

I always thought it was increasing at a steady rate,  but naturally that would mean the farther something is away, the faster it would move away simply because the space in between is expanding.

is that it? 

or am i to believe that things are expanding faster than they used to?   as in,  lets say you had two galaxies a certain distance from one another. would they move away from eachother at a faster rate now than they would have a billion years ago?  

 

The acceleration of the expansion was discovered a couple of decades ago.  This was due to a study involving Type 1A supernovae.  We had learned that these types of supernovae always reach the same peak brightness.  This made them a "standard candle".  You could tell how far away one was by measuring how bright it appeared to us.

It never really was thought that the rate of the expansion of the universe was constant.  That is because the gravitational attraction of everything in it was expected to slow the expansion over time.   The one thing we didn't know was if this would be enough to eve completely stop the expansion.

The study's original intent was to answer this question.   Now the supernovae standard candle gave us a measure of a galaxy's distance and red-shift its speed of recession.  The other factor that comes into play is that the further the galaxy is from us, the further back in time the light we are seeing left it.   We are seeing a red-shift based on what was happening at that time.   If the universe was expanding at a constant rate, then the ratio between distance and red-shift would be the same for all galaxies no matter what distance they are.  If the expansion were slowing with time, it would not be a constant ratio with the ratio changing in a particular manner.  The magnitude of this change would give you a measure of the rate at which the universe's expansion.

The study found such a deviation, but in the opposite direction than what they expected.  The red-shift/distance ratio changed in a manner that indicated that the universe was expanding slower in the past than it is now.  Instead of slowing down as time went by, it was speeding up.  "Dark energy" was the term coined for whatever was driving this acceleration.  The exact nature of Dark energy is still a mystery, and there are competing hypothesizes as to what it actually is. 

[boo]

On 10/16/2019 at 4:59 PM, Janus said:

The acceleration of the expansion was discovered a couple of decades ago.  This was due to a study involving Type 1A supernovae.  We had learned that these types of supernovae always reach the same peak brightness.  This made them a "standard candle".  You could tell how far away one was by measuring how bright it appeared to us.

It never really was thought that the rate of the expansion of the universe was constant.  That is because the gravitational attraction of everything in it was expected to slow the expansion over time.   The one thing we didn't know was if this would be enough to eve completely stop the expansion.

The study's original intent was to answer this question.   Now the supernovae standard candle gave us a measure of a galaxy's distance and red-shift its speed of recession.  The other factor that comes into play is that the further the galaxy is from us, the further back in time the light we are seeing left it.   We are seeing a red-shift based on what was happening at that time.   If the universe was expanding at a constant rate, then the ratio between distance and red-shift would be the same for all galaxies no matter what distance they are.  If the expansion were slowing with time, it would not be a constant ratio with the ratio changing in a particular manner.  The magnitude of this change would give you a measure of the rate at which the universe's expansion.

The study found such a deviation, but in the opposite direction than what they expected.  The red-shift/distance ratio changed in a manner that indicated that the universe was expanding slower in the past than it is now.  Instead of slowing down as time went by, it was speeding up.  "Dark energy" was the term coined for whatever was driving this acceleration.  The exact nature of Dark energy is still a mystery, and there are competing hypothesizes as to what it actually is. 

thats fascinating

could it be possible that the faster rate at which things are expanding, is a natural result of the fact that the universe is simply bigger now than it was before?  or is it moving even faster still?

 

Edited October 19, 2019 by boo

[Janus]

5 hours ago, boo said:

thats fascinating

could it be possible that the faster rate at which things are expanding, is a natural result of the fact that the universe is simply bigger now than it was before?  or is it moving even faster still?

 

No. If you plot distance against measured red-shift you get a straight line if the universe has been expanding at a constant rate. If the expansion had been slowing down, this plot would curve slightly in one direction (this is what the study expected to find, and they were interested in how much it curved.)   If the rate of expansion where speeding up over time, the plot would curve in the other direction ( which is what the study actually found).

It is the equivalent of tossing a ball up into the air on a small moon which you don't know the mass or radius of.  As the ball climbs, it loses speed.  By noting how fast it loses speed, you can tell if the ball is moving at escape velocity or not. You can tell whether it will eventually fall back down to you or will just keep climbing away.   What you would not expect is for the ball to pick up speed as it climbed.  This is essentially what the study found. 

It was because this result was so contrary to what was expected that this study became so important. It's not that it was "unnatural"( anything the Universe does is "natural"), but more that it uncovered something about the universe that we did not suspect. The most exciting words in science are not "Aha, just as I expected!", but "Huh, that's odd!"