8 ms·

As a layman, one thing I don't understand is why we suppose there has to be dark matter and energy in the universe. I understand that dark matter is used to ex

by mstank 10y ago

As a layman, one thing I don't understand is why we suppose there has to be dark matter and energy in the universe. I understand that dark matter is used to explain gravitational irregularities when observing galaxies. However, saying 'there is x amount of unobservable stuff there' in order to make our calculations correct seems like lazy science. Are there any theories that don't include dark matter or energy? Can gravity function differently depending on its location (space or time) in the universe?

empath75 10y ago
https://en.wikipedia.org/wiki/Dark_matter#Alternative_theories https://en.wikipedia.org/wiki/Dark_matter#Alternative_theori... Yes. Hardly anyone takes them seriously, though, for a variety of reasons.
Manishearth 10y ago
> However, saying 'there is x amount of unobservable stuff there' in order to make our calculations correct seems like lazy science. That's not how this works. Dark matter and energy are placeholders for anything that can explain the discrepancy. The most likely explanation for dark matter is just undiscovered particles, but changing the physics of it to make the calculations come out right is also something that people do. There's not really a difference between the two either. Post-QFT the concept of a particle is much blurrier than before, and "changing the physics" might just involve introducing a new background field that behaves differently in different places based on its concentration. In essence, you have introduced a new particle. Please note that while it feels like lazy science, coming up with alternate theories that have the same success and rigor as QFT is very hard. Its not that they are hanging up their pencils and declaring premature victory. It is that literally the best description of physics out there to date doesn't explain this, and within that description yet-to-be-observed particles is its best bet. They are working on better explanations, but Rome wasn't built in a day.
- teamonkey 10y ago
  Here's an interesting theory. https://astronomynow.com/2016/06/16/did-gravitational-wave-detector-find-dark-matter/ https://astronomynow.com/2016/06/16/did-gravitational-wave-d... The gravitational waves we're detecting come from black holes, but the readings we have from LIGO suggest that they most likely come from 'primordial' black holes formed at the birth of the universe. If many of these black holes were formed they would be distributed much as MACHO theory predicts. 'Normal' black holes have very specific mass profiles so were generally ruled out as MACHOs, although obviously 'dark matter = black holes' has had a lot of consideration.
- iaw 10y ago
  "The most likely explanation for dark matter is just undiscovered particles" <--- all valid points but I'd argue that dark matter may actually be strictly the result of yet-to-be-understood underlying fields. e.g. dark matter may not have any particles itself but could be the gravitational of spacetime in a sense Just a nit.
  - johncolanduoni 10y ago
    Unless the field is not quantized, this is not a distinction. Electrons, protons, etc. are a fields as well.
    - iaw 10y ago
      The fields that results in the Casimir effect are non-quantized, correct?
      - johncolanduoni 10y ago
        On the contrary, Casimir forces only arise because the field is quantized.
  - Manishearth 10y ago
    I said exactly this in my comment. There is little distinction between particle and field in this context.
jwatte 10y ago
Prediction: Dark matter is the emissions/pollution from the EM-drives of previous and remote civilizations. Two questions solved in one answer ;-)
- drdeca 10y ago
  If you look at the more recent EM drive stuff, I think it looks like the evidence it turning against it somewhat more clearly now?
  - eLiza1 10y ago
    What evidence is that?
- Tyr42 10y ago
  Why is it found more densely at the outside of galaxies? What does it say about those civs?
wyager 10y ago
Dark energy is experimentally confirmed to exist. It's existence isn't a problem for modern physics; the problem is that it's many orders of magnitude smaller than predicted.
- johncolanduoni 10y ago
  Whoa whoa whoa. It's experimentally confirmed that the discrepancy between what GR predicts the cosmological structure of the universe should look like and what our measurements can glean fits a uniform energy field with a specific density that accelerates expansion. It's never been observed in any other capacity, and dark energy is generally just taken as a way of saying "whatever makes this particular term necessary in GR at certain scales". There's not even a consensus on how to fit it into GR; the simplest and original solution is the cosmological constant, but there's also the approach of treating it as a field that is dynamic (can vary in space and time) in principle but just happens to be too uniform for us to tell. That doesn't even take into account the fact that this could be a sign that GR is simply missing something when it comes to the large scale structure of the cosmos. We've ruled out some proposed alternate theories experimentally but not all, and that's just the proposed ones.
  - wyager 10y ago
    >It's never been observed in any other capacity, The Casimir effect and Lamb shift are strong experimental evidence of vacuum energy, which is mathematically equivalent to Einstein's "cosmological constant".
    - johncolanduoni 10y ago
      How are they remotely mathematically equivalent? The Casimir effect/Lamb shift calculations do not need any input from the cosmological constant's magnitude or anything equivalent. You can calculate them with naught but the constants you need to fix the form of QED. If you try to go the reverse direction and claim that vacuum fluctuations are what the cosmological constant is really about, like you said you have to explain a massive (orders of magnitude!) discrepancy. Considering that the cosmological constant is nothing more than a magnitude as far as we can tell, a theory about a link that gives the wrong magnitude is completely useless. You can draw a parallel between them conceptually, but that connection is about as solid as the connection between the cosmological constant and any other isotropic scalar field like the Higgs. This is another speculative explanation, but again there is no theory that actually manages to explain the value.
pdkl95 10y ago
> why we suppose there has to be dark matter and energy in the universe. PBS Space Time has a set of short videos[1] that do a very good job of explaining dark {matter,energy}. It starts with Einstein's field equations and GR, then walks through what the Friedmann equations say about the expansion of the universe. [1] https://www.youtube.com/watch?v=xZTb6sfHEX8 https://www.youtube.com/watch?v=xZTb6sfHEX8 (and the next few videos: https://www.youtube.com/pbsspacetime/videos https://www.youtube.com/pbsspacetime/videos )
- dwaltrip 10y ago
  For those who haven't seen yet any of the PBS Space Time videos and are interested in physics and cosmology, it is really well done. I recommend checking it out. Each episode is pretty short, entertaining, and jam packed with well explained information. I'm now hooked on the series, and feel like I am more informed about our latest understanding of physics and the universe.
ubernostrum 10y ago
However, saying 'there is x amount of unobservable stuff there' in order to make our calculations correct seems like lazy science. "Dark matter" and "dark energy" don't refer to unobservable things, just to things whose existence we suspect but which we have not yet been able to observe. The planet Neptune was once in that category, as was the neutrino, and both of them were predicted "backwards" from a discrepancy between theory and observation (the observed orbit of Uranus didn't match what theory predicted, and it turned out the gravity of as-yet-unobserved Neptune was the cause; beta decay appeared to violate conservation laws, and it turned out the as-yet-undetected neutrino accounted for the "missing" energy and angular momentum).
Tyr42 10y ago
I don't know anything about dark energy, but I can talk about dark matter. So, lets look at the galaxy rotational curve data[1]. (Go look at the picture in the top right.) Galaxies don't spin like a record, because they aren't stiff. Instead, the stuff spins at different speeds. Also, galaxies are not like the sun and planets, where almost all the mass is at the centre, but rather it's spread out over the disk. Now, we can try and predict how the mass is spread out, based on the size and colour of the stars we can see, and use that to get one prediction of how the galaxy should rotate. We can also measure red/blue shift from stars, and use that to try and actually measure how the galaxy is rotating. The problem is that these two don't match up. Which is a problem. So, we're trying to find ways to explain why they don't match up. Now, one way to do this would be to note that if there was X amount of matter that we didn't account for when we only looked at the stars, then the curve would in fact match up. Since this matter isn't stars, and doesn't give off light, it was called dark matter. And it seems more likely that there's a bunch of mass we can't see than that gravity works weird. [1]: https://en.wikipedia.org/wiki/Galaxy_rotation_curve https://en.wikipedia.org/wiki/Galaxy_rotation_curve
Avshalom 10y ago
It's worth point out that we have additional direct evidence of dark matter besides the rotation problem, like most thing's it's not perfectly cut an dry smoking gun evidence but: https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Abell_520 https://en.wikipedia.org/wiki/Abell_520

by mstank 10y ago

empath75 10y ago

https://en.wikipedia.org/wiki/Dark_matter#Alternative_theories https://en.wikipedia.org/wiki/Dark_matter#Alternative_theori... Yes. Hardly anyone takes them seriously, though, for a variety of reasons.

Manishearth 10y ago

> However, saying 'there is x amount of unobservable stuff there' in order to make our calculations correct seems like lazy science. That's not how this works. Dark matter and energy are placeholders for anything that can explain the discrepancy. The most likely explanation for dark matter is just undiscovered particles, but changing the physics of it to make the calculations come out right is also something that people do. There's not really a difference between the two either. Post-QFT the concept of a particle is much blurrier than before, and "changing the physics" might just involve introducing a new background field that behaves differently in different places based on its concentration. In essence, you have introduced a new particle. Please note that while it feels like lazy science, coming up with alternate theories that have the same success and rigor as QFT is very hard. Its not that they are hanging up their pencils and declaring premature victory. It is that literally the best description of physics out there to date doesn't explain this, and within that description yet-to-be-observed particles is its best bet. They are working on better explanations, but Rome wasn't built in a day.

teamonkey 10y ago
Here's an interesting theory. https://astronomynow.com/2016/06/16/did-gravitational-wave-detector-find-dark-matter/ https://astronomynow.com/2016/06/16/did-gravitational-wave-d... The gravitational waves we're detecting come from black holes, but the readings we have from LIGO suggest that they most likely come from 'primordial' black holes formed at the birth of the universe. If many of these black holes were formed they would be distributed much as MACHO theory predicts. 'Normal' black holes have very specific mass profiles so were generally ruled out as MACHOs, although obviously 'dark matter = black holes' has had a lot of consideration.
iaw 10y ago
"The most likely explanation for dark matter is just undiscovered particles" <--- all valid points but I'd argue that dark matter may actually be strictly the result of yet-to-be-understood underlying fields. e.g. dark matter may not have any particles itself but could be the gravitational of spacetime in a sense Just a nit.
- johncolanduoni 10y ago
  Unless the field is not quantized, this is not a distinction. Electrons, protons, etc. are a fields as well.
  - iaw 10y ago
    The fields that results in the Casimir effect are non-quantized, correct?
    - johncolanduoni 10y ago
      On the contrary, Casimir forces only arise because the field is quantized.
- Manishearth 10y ago
  I said exactly this in my comment. There is little distinction between particle and field in this context.

jwatte 10y ago

Prediction: Dark matter is the emissions/pollution from the EM-drives of previous and remote civilizations. Two questions solved in one answer ;-)

drdeca 10y ago
If you look at the more recent EM drive stuff, I think it looks like the evidence it turning against it somewhat more clearly now?
- eLiza1 10y ago
  What evidence is that?
Tyr42 10y ago
Why is it found more densely at the outside of galaxies? What does it say about those civs?

wyager 10y ago

Dark energy is experimentally confirmed to exist. It's existence isn't a problem for modern physics; the problem is that it's many orders of magnitude smaller than predicted.

johncolanduoni 10y ago
Whoa whoa whoa. It's experimentally confirmed that the discrepancy between what GR predicts the cosmological structure of the universe should look like and what our measurements can glean fits a uniform energy field with a specific density that accelerates expansion. It's never been observed in any other capacity, and dark energy is generally just taken as a way of saying "whatever makes this particular term necessary in GR at certain scales". There's not even a consensus on how to fit it into GR; the simplest and original solution is the cosmological constant, but there's also the approach of treating it as a field that is dynamic (can vary in space and time) in principle but just happens to be too uniform for us to tell. That doesn't even take into account the fact that this could be a sign that GR is simply missing something when it comes to the large scale structure of the cosmos. We've ruled out some proposed alternate theories experimentally but not all, and that's just the proposed ones.
- wyager 10y ago
  >It's never been observed in any other capacity, The Casimir effect and Lamb shift are strong experimental evidence of vacuum energy, which is mathematically equivalent to Einstein's "cosmological constant".
  - johncolanduoni 10y ago
    How are they remotely mathematically equivalent? The Casimir effect/Lamb shift calculations do not need any input from the cosmological constant's magnitude or anything equivalent. You can calculate them with naught but the constants you need to fix the form of QED. If you try to go the reverse direction and claim that vacuum fluctuations are what the cosmological constant is really about, like you said you have to explain a massive (orders of magnitude!) discrepancy. Considering that the cosmological constant is nothing more than a magnitude as far as we can tell, a theory about a link that gives the wrong magnitude is completely useless. You can draw a parallel between them conceptually, but that connection is about as solid as the connection between the cosmological constant and any other isotropic scalar field like the Higgs. This is another speculative explanation, but again there is no theory that actually manages to explain the value.

pdkl95 10y ago

> why we suppose there has to be dark matter and energy in the universe. PBS Space Time has a set of short videos[1] that do a very good job of explaining dark {matter,energy}. It starts with Einstein's field equations and GR, then walks through what the Friedmann equations say about the expansion of the universe. [1] https://www.youtube.com/watch?v=xZTb6sfHEX8 https://www.youtube.com/watch?v=xZTb6sfHEX8 (and the next few videos: https://www.youtube.com/pbsspacetime/videos https://www.youtube.com/pbsspacetime/videos )

dwaltrip 10y ago
For those who haven't seen yet any of the PBS Space Time videos and are interested in physics and cosmology, it is really well done. I recommend checking it out. Each episode is pretty short, entertaining, and jam packed with well explained information. I'm now hooked on the series, and feel like I am more informed about our latest understanding of physics and the universe.

ubernostrum 10y ago

However, saying 'there is x amount of unobservable stuff there' in order to make our calculations correct seems like lazy science. "Dark matter" and "dark energy" don't refer to unobservable things, just to things whose existence we suspect but which we have not yet been able to observe. The planet Neptune was once in that category, as was the neutrino, and both of them were predicted "backwards" from a discrepancy between theory and observation (the observed orbit of Uranus didn't match what theory predicted, and it turned out the gravity of as-yet-unobserved Neptune was the cause; beta decay appeared to violate conservation laws, and it turned out the as-yet-undetected neutrino accounted for the "missing" energy and angular momentum).

Tyr42 10y ago

I don't know anything about dark energy, but I can talk about dark matter. So, lets look at the galaxy rotational curve data[1]. (Go look at the picture in the top right.) Galaxies don't spin like a record, because they aren't stiff. Instead, the stuff spins at different speeds. Also, galaxies are not like the sun and planets, where almost all the mass is at the centre, but rather it's spread out over the disk. Now, we can try and predict how the mass is spread out, based on the size and colour of the stars we can see, and use that to get one prediction of how the galaxy should rotate. We can also measure red/blue shift from stars, and use that to try and actually measure how the galaxy is rotating. The problem is that these two don't match up. Which is a problem. So, we're trying to find ways to explain why they don't match up. Now, one way to do this would be to note that if there was X amount of matter that we didn't account for when we only looked at the stars, then the curve would in fact match up. Since this matter isn't stars, and doesn't give off light, it was called dark matter. And it seems more likely that there's a bunch of mass we can't see than that gravity works weird. [1]: https://en.wikipedia.org/wiki/Galaxy_rotation_curve https://en.wikipedia.org/wiki/Galaxy_rotation_curve

Avshalom 10y ago

It's worth point out that we have additional direct evidence of dark matter besides the rotation problem, like most thing's it's not perfectly cut an dry smoking gun evidence but: https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Abell_520 https://en.wikipedia.org/wiki/Abell_520