Do we need Dark Matter or are we just time blind

In summary, the stars close to the Black Hole at the center of the Milky Way are moving faster than predicted by classical theory. To explain this, Dark matter has been "invented" to explain the higher than expected gravitational force necessary to produce these higher velocities. However, time in the extreme gravitational fields close to a black hole is known to slow down. So, are these stars actually moving at this unusual speed or is the difference in time frame making it appear to us that they are moving faster than they should be? If we can explain the observed anomalies in velocity by the difference in time frames between the star and the earth, do we still need Dark Matter? More worryingly
  • #1
Bruce Wilson
5
0
As humans, we intuitively tend to interpret the things we see in our local time frame. However, the rate at which time flows at any point (or time) in the Universe is affected by the strength of the gravitational field at that point (or time). Hence when we see distant events through our telescopes and interpret them in the lab in our own time frame (time rate), errors will occur.

Nearly all the methods of measuring a star’s tangential velocity involve taking a number of measurements at known increments of time. (Red-shifting only measures radial velocity). The increment of time is measured in Earth time units since it is difficult to remotely calculate the star’s time frame Hence we are always viewing the velocity of the star in the time frame of the earth. This will produce velocities which appear higher than the actual local velocities when viewing a star with mass greater than the earth. This is a type of time blindness.

For example, the stars close to the Black Hole at the centre of the Milky Way are observed to be moving faster than predicted by classical theory To explain this, Dark matter has been “invented” to explain the higher than expected gravitational force necessary to produce these higher velocities.

However, time in the extreme gravitational fields close to a black hole is known to slow down. So, are these stars actually moving at this unusual speed or is the difference in time frame making it appear to us that they are moving faster than they should be? If we can explain the observed anomalies in velocity by the difference in time frames between the star and the earth, do we still need Dark Matter?

More worryingly, if time frame difference is not routinely corrected for, are we looking at a distorted picture of the Universe? The size of the error could potentially be very large. For example, the time rate close to a black hole is thought to approach zero and hence the corresponding velocity in our time frame would approach infinity. The data comparing actual speeds (measured in our time frame) of stars close to black holes with the speeds calculated from Newtonian theory could be used to calculate how much of the error is caused by changes in time frame.

Time Rate Mapping
A very useful tool would be if time frames could be mapped. The ratio of local time rate to GMT time rate could be used as a parameter. This could be done from gravitational lensing and by observing the speed of rotation of stars round black holes and galaxies round clumps of galaxies. This is similar to the methods used to map dark matter. This combined with the fact that dark matter appears to attract galaxies, which are areas of high gravity and slow time, could mean that a slow time map would be very similar to a dark matter map. It might even be possible to use existing dark matter maps to produce an approximate time correction when measuring the tangential velocity of stars.
 
Last edited by a moderator:
Space news on Phys.org
  • #2
Bruce Wilson said:
the stars close to the Black Hole at the centre of the Milky Way are observed to be moving faster than predicted by classical theory

No, actually it's the other way around. The stars close to the center are moving the way we would expect from the matter distribution we can see. The stars further from the center are the ones that are moving faster than we would expect from the matter distribution we can see. So your proposal, even if it were correct as regards the magnitude of the time dilation effect (which it isn't--see below), would not explain what we actually see.

Bruce Wilson said:
are these stars actually moving at this unusual speed or is the difference in time frame making it appear to us that they are moving faster than they should be?

Have you actually done the math to see how close these stars would need to be to the central black hole for any such time dilation to be significant? (Hint: it's way closer to the hole than any of the stars actually are.)
 
  • Like
Likes White_Wolf
  • #3
Yes we need dark matter. The effect of gravitation on time is miniscule in the relevant measurements and could anyway be easily corrected for if this was not the case.
 
  • #4
Orodruin said:
Yes we need dark matter. The effect of gravitation on time is miniscule in the relevant measurements and could anyway be easily corrected for if this was not the case.
To expand on this a little, gravitational time dilation is only significant very close to extremely dense objects (such as neutron stars or black holes). The fact that the "missing mass" is more of a problem further away from the centers of galaxies indicates that it can't be explained away through time dilation.

Also, the existence of dark matter is supported by a wide body of evidence. Here's a blog post describing one particularly evocative piece of evidence:
http://www.preposterousuniverse.com/blog/2006/08/21/dark-matter-exists/
 
  • #6
Thanks everyone for your information its been really useful.
 
  • #7
PeterDonis said:
The stars close to the center are moving the way we would expect from the matter distribution we can see. The stars further from the center are the ones that are moving faster than we would expect from the matter distribution we can see.

How do we know what to expect, without knowing the depth our gravity well, relative to the depth of the gravity well of the galaxy we're observing? As a hypothetical example, suppose our closeness to the barycenter of our local group of galaxies makes clocks close to the center of the galaxy we're observing run at the same rate as ours as we measure.
 
  • #8
rundal said:
How do we know what to expect, without knowing the depth our gravity well, relative to the depth of the gravity well of the galaxy we're observing?

The time dilation due to all of the gravity wells involved is way too small to affect any of these observations. The Milky Way's gravity well, for example, results in a time dilation of about one part in 100,000. Other galaxies are similar. The galaxy rotation curve anomalies that dark matter is postulated to explain are orders of magnitude larger than that.
 
  • Like
Likes rundal
  • #9
PeterDonis said:
The Milky Way's gravity well, for example, results in a time dilation of about one part in 100,000.

I see. Just entertaining ideas here, how can we know that for sure? Presumably to get that one part in 100,000 result you need the mass, and to get the mass we measure orbital velocity. But maybe the orbital velocity is slowed by significant gravitational time dilation, so we're fooled into thinking there's much less mass. What rules out that possibility?

Edit: I see you gave an answer to this above already. I wasn't thinking about a black hole at the center of the galaxy, but rather a large quantity of densely packed stars (perhaps mixed in with black holes, I suppose) orbiting the center.
 
Last edited:
  • #10
rundal said:
I see. Just entertaining ideas here, how can we know that for sure? Presumably to get that one part in 100,000 result you need the mass, and to get the mass we measure orbital velocity. But maybe the orbital velocity is slowed by significant gravitational time dilation, so we're fooled into thinking there's much less mass. What rules out that possibility?
Gravitational time dilation is an incredibly tiny effect in nearly all cases. Basically, if you're not talking about the area immediately surrounding a black hole, you can pretty safely ignore gravitational time dilation in most contexts.
 
  • #11
I'd still like to know how we're sure it's a tiny effect in this case. What if for every star in the galaxy's central bulge that we see, there are 1000 black holes, or highly compact / highly redshifted stars, all orbiting the galaxy's center? I suppose we could rule that out because we don't see evidence of gravitational lensing. Is that how it's ruled out? Yes the gravitational time dilation might be significant only in the area immediately surrounding the bulge, but it seems that's all it would take to measure a slower (than locally measured) orbital velocity for a star in the area, and hence greatly underestimate the mass within the bulge.
 
  • #12
rundal said:
I'd still like to know how we're sure it's a tiny effect in this case. What if for every star in the galaxy's central bulge that we see, there are 1000 black holes, or highly compact / highly redshifted stars, all orbiting the galaxy's center? I suppose we could rule that out because we don't see evidence of gravitational lensing. Is that how it's ruled out? Yes the gravitational time dilation might be significant only in the area immediately surrounding the bulge, but it seems that's all it would take to measure a slower (than locally measured) orbital velocity for a star in the area, and hence greatly underestimate the mass within the bulge.
So you would remove the need for dark matter by introducing copious amounts of matter that cannot be seen? Do you see the irony in this?

To be perfectly frank, your argumentation is based on suppositions and word play. What people are telling you is generally supported by the mathematical predictions of general relativity which has been shown to be in very good agreement with observations. If you want to dispute this, do the math. (But note that PF is not the place for personal theories or original research. The proper place for that is in a reputable peer reviewed journal (if accepted).
 
  • #13
rundal said:
I'd still like to know how we're sure it's a tiny effect in this case. What if for every star in the galaxy's central bulge that we see, there are 1000 black holes, or highly compact / highly redshifted stars, all orbiting the galaxy's center? I suppose we could rule that out because we don't see evidence of gravitational lensing. Is that how it's ruled out? Yes the gravitational time dilation might be significant only in the area immediately surrounding the bulge, but it seems that's all it would take to measure a slower (than locally measured) orbital velocity for a star in the area, and hence greatly underestimate the mass within the bulge.
In order for time dilation around a black hole to reach 1%, you'd need to be about 100 times the Schwarzschild radius away. That may sound large, but bear in mind that black holes are tiny. A black hole with the mass of the Earth would be about the size of a golf ball. A black hole the mass of the Sun would only have a radius of about 3000 meters. That means that you'd have to be within 300km of a black hole the mass of the Sun to have time dilation greater than 1%. Contrast this with typical distances between astronomical objects which are often measured in the trillions of kilometers (1 light year = 9.4 trillion kilometers).
 
  • #14
Orodruin said:
So you would remove the need for dark matter by introducing copious amounts of matter that cannot be seen? Do you see the irony in this?

The current thinking is that the flat rotation curves are accurate. I'm indirectly asking about the possibility that maybe the curves aren't flat if they were completely locally measured. The copious amounts of matter I'm talking about wouldn't be distributed the same as is currently thought it must be.

I'm not proposing a personal theory. I'm asking how some possibilities are ruled out.
 
  • #15
rundal said:
The current thinking is that the flat rotation curves are accurate. I'm indirectly asking about the possibility that maybe the curves aren't flat if they were completely locally measured. The copious amounts of matter I'm talking about wouldn't be distributed the same as is currently thought it must be.

I'm not proposing a personal theory. I'm asking how some possibilities are ruled out.

This does not change the fact that what you are describing just cannot be a viable solution if you actually do the maths - as has been explained by many posters in this thread. GR just does not behave the way you would need it to.
 
  • #16
Chalnoth said:
Contrast this with typical distances between astronomical objects which are often measured in the trillions of kilometers (1 light year = 9.4 trillion kilometers).

Okay, so I guess if the central bulge was much more dense, say distances between astronomical objects averaging 0.05 light years, with 10000 dark stars (maybe black holes) for every visible star, we'd definitely notice this, right? We'd notice gravitational lensing, visible stars being dimmed or blocked as dark stars pass in front of them, or other obvious things?

Yes, a black hole the mass of the Sun would only have a radius of about 3000 meters, but I'm thinking about black holes the size of the Sun, with 10000 of them for every visible star in the central bulge. Just to entertain the idea and see how we rule out that possibility.
 
  • #17
Orodruin said:
This does not change the fact that what you are describing just cannot be a viable solution if you actually do the maths - as has been explained by many posters in this thread. GR just does not behave the way you would need it to.

I can do the math in my head, to know that if enough dark mass is packed into the galaxy's central bulge, it's possible to inaccurately measure the orbital velocity of a star to any degree. That's why I'm asking how this possibility is ruled out. I'm not the OP. My questions are within the context of GR.
 
  • #18
Like anything else that enjoys scientific acceptance, it's not due to one measurement. Fred Zwicky hypothesized about the existence of dark matter because of the velocity of entire galaxies within a cluster, not the stars themselves. Vera Wang then proposed the same hypothesis when noticing that the stars don't orbit the way they were expected to. Gravitational lending has also allowed us to map where the anomalies are and they don't always match up to where the matter is. The bullet cluster contains galaxies that smashed through each other. The stars of course didn't collide so they passed right through each other. The loose gas and dust did, so it got ripped away from the galaxies and stayed in between. The dark matter stayed with the stars, if it were regular matter, it should have had friction with itself and stayed with the gas.
 
  • #19
newjerseyrunner said:
The dark matter stayed with the stars, if it were regular matter, it should have had friction with itself and stayed with the gas.

Can you elaborate on this? What is the basis of the assumption that the dark matter can't be dark stars or black holes, for the Bullet Cluster? In which case it seems we could expect them to stay with the stars and not the gas.
 
  • #20
rundal said:
Okay, so I guess if the central bulge was much more dense, say distances between astronomical objects averaging 0.05 light years, with 10000 dark stars (maybe black holes) for every visible star, we'd definitely notice this, right? We'd notice gravitational lensing, visible stars being dimmed or blocked as dark stars pass in front of them, or other obvious things?
Can't work. Just adding a bunch of extra mass to the center of the galaxy won't flatten the rotation curves. You need matter distributed smoothly throughout the galaxy.

Also, your scenario still has typical separations between "dark stars" at 20 billion kilometers, which is much too far to experience substantial time dilation for anything but a supermassive black hole.
 
  • #21
rundal said:
Can you elaborate on this? What is the basis of the assumption that the dark matter can't be dark stars or black holes, for the Bullet Cluster? In which case it seems we could expect them to stay with the stars and not the gas.
Black stars do not exist in the universe, it sinply isn't old enough. Black holes have been ruled out by looking for micro lensing. calculations show 6 times the mass of dark matter to regular matter. If they were stars, they'd be everywhere, same with black holes. We've also ruled out rogue planets. We can also rule out tiny black holes that wouldn't create micro lending because they should have exploded by now, and we've never seen a black hole explode as far as we know.
 
  • #22
rundal said:
Can you elaborate on this? What is the basis of the assumption that the dark matter can't be dark stars or black holes, for the Bullet Cluster? In which case it seems we could expect them to stay with the stars and not the gas.
The impact of dark matter is clearly visible in the CMB, which was emitted long before any stars formed, let alone black holes.
 
  • #23
Chalnoth said:
The impact of dark matter is clearly visible in the CMB, which was emitted long before any stars formed, let alone black holes.
You mean stellar black holes? Weren't primordial black holes created in the first second?

Primordial black holes have also been ruled out for dark matter, (I remember personally asking this about a year ago.) that's not why I brought it up, but for completeness I'm fairly certain black holes should have existed when the CMB was created 300K years into time.
 
  • #24
newjerseyrunner said:
You mean stellar black holes? Weren't primordial black holes created in the first second?

Primordial black holes have also been ruled out for dark matter, (I remember personally asking this about a year ago.) that's not why I brought it up, but for completeness I'm fairly certain black holes should have existed when the CMB was created 300K years into time.
I don't think there's any reason currently to believe that there are any primordial black holes at all. They should be considered an exotic possibility with no supporting evidence.

The main issue, in my mind, is that most potential masses for such black holes would have resulted in relics which would have been readily-detectable with current observations. The possibilities for significant numbers of primordial black holes only exist in some quite narrow mass ranges. Primordial black holes aren't completely impossible, but it's not at all easy to come up with reasonable models where they exist in noticeable quantities. There's a good chance that all mass ranges will be ruled out within a few years.

Here's one paper that goes over some of the evidence:
https://arxiv.org/abs/1607.06077
 
  • Like
Likes newjerseyrunner
  • #25
Chalnoth said:
Also, your scenario still has typical separations between "dark stars" at 20 billion kilometers, which is much too far to experience substantial time dilation for anything but a supermassive black hole.

Okay, I can see that's way too sparse to experience substantial time dilation. If I keep narrowing the separation between objects, if only by growing the dark stars or black holes by adding mass, eventually I guess it'd be obvious there's dark baryonic matter there, like the visible stars would be routinely blocked by the dark stars 1000+ times the size of the visible stars. Which we don't observe.

Just adding a bunch of extra mass to the center of the galaxy won't flatten the rotation curves. You need matter distributed smoothly throughout the galaxy.

I'm not seeing this yet. Are you saying that gravitational time dilation can't possibly flatten the curve, or are you saying that significant gravitational time dilation is ruled out?
 
  • #26
rundal said:
I'm not seeing this yet. Are you saying that gravitational time dilation can't possibly flatten the curve, or are you saying that significant gravitational time dilation is ruled out?
I'm saying that adding a bunch of mass to the center will result in a steeper rotation curve, not a shallower one.
 
  • #27
newjerseyrunner said:
Vera Wang

Vera Rubin was the astronomer. Vera Wang is the fashion designer.
 
  • Like
Likes phinds, stoomart and newjerseyrunner
  • #28
Haha, oops.
 
  • #29
Apologies for my absence from the discussions and thank you for your interest in the ideas. I accept that time dilation is too small to have any practical affect on astronomical measurements.

However, I still think that the concept of viewing an event from a different time frame is one which should always be considered.
As a frivolous example, imagine driving your car slowly up to a brick wall until you just touched it. If the event was recorded using time lapse photography, the resulting video might show that it was perfectly safe to drive into a brick wall at 100mph.
During the big bang, the entire mass/energy of the universe was concentrated in a tiny “pinprick”.

The gravitational forces in this pinprick must have been enormous and the time dilation appreciable.

Hence the velocity and hence violence of the initial expansion may not have been as great as we now think when viewing it in our time frame.
Hence, perhaps the universe did not expand faster than light speed, perhaps the expansion was not violent enough to produce significant gravitational waves and according to the particles involved, the universe is older than 13.7 billion years.
 
  • #30
Bruce Wilson said:
The gravitational forces in this pinprick must have been enormous and the time dilation appreciable.

No, they weren't. The spacetime curvature in the pinprick was enormous (because the density of stress-energy was enormous), but that spacetime curvature showed up as very rapid expansion, not as "gravitational forces". And there was no gravitational time dilation because the density was the same everywhere, so the spacetime curvature at a given instant of time was the same everywhere.

Bruce Wilson said:
Hence the velocity and hence violence of the initial expansion may not have been as great as we now think when viewing it in our time frame.

Hence, perhaps the universe did not expand faster than light speed, perhaps the expansion was not violent enough to produce significant gravitational waves and according to the particles involved, the universe is older than 13.7 billion years.

These speculations are incorrect, because they are based on a mistaken premise. See above.
 
  • #31
The baryon mass fraction of the universe has been measured with great precision by a number of studies [Planck, WMAP, BAO] which rules out any significant unaccounted contributions from black holes. For further discussion see https://ned.ipac.caltech.edu/level5/Peebles2/P5_2.html
 

Related to Do we need Dark Matter or are we just time blind

1. What is Dark Matter and why is it important in science?

Dark Matter is a hypothetical form of matter that does not interact with light or other forms of electromagnetic radiation. It is believed to make up about 85% of the total matter in the universe and is crucial in explaining the structure and evolution of the universe.

2. How do we know that Dark Matter exists?

Scientists have observed the effects of Dark Matter on the motion of stars and galaxies. The gravitational pull of Dark Matter is necessary to explain the observed rotation curves of galaxies and the bending of light in gravitational lensing.

3. Can Dark Matter be detected or observed?

Currently, Dark Matter cannot be directly detected or observed. It does not emit or absorb light, making it invisible to telescopes. However, scientists are working on experiments to indirectly detect Dark Matter through its interactions with other particles.

4. Is it possible that our understanding of gravity is incomplete and Dark Matter is not needed?

While it is always possible that our understanding of gravity may be incomplete, the evidence for Dark Matter is strong and has been supported by multiple independent observations. Additionally, alternative theories of gravity have not been able to fully explain the observed effects of Dark Matter.

5. Could Dark Matter be a result of our limited understanding of time?

There is currently no evidence to suggest that Dark Matter is a result of our limited understanding of time. The concept of Dark Matter has been extensively studied and is supported by a vast amount of observational evidence. While our understanding of time may continue to evolve, it is unlikely to change the need for Dark Matter in explaining the structure and evolution of the universe.

Similar threads

Replies
16
Views
1K
Replies
10
Views
1K
Replies
20
Views
2K
Replies
9
Views
1K
  • Astronomy and Astrophysics
Replies
13
Views
605
  • Special and General Relativity
Replies
12
Views
496
  • Astronomy and Astrophysics
Replies
8
Views
271
  • Cosmology
Replies
18
Views
2K
  • Cosmology
Replies
13
Views
3K
Back
Top