Dark matter/energy, why must it be there?

[Pedro Batista]

Hey everyone. This question is coming from someone who is just a science enthusiast, with no training whatsoever in physics, and the enquiry comes from genuine curiosity.

To my understanding, dark matter and energy are concepts created after we made some observations that we could not account for just by looking at what we see. Example, the gravitational pull from the matter we observe shouldn't allow galaxies to be pulling away from each other, so, there must be something (dark energy) that is doing it.

My question is, why do we assume that this dark stuff is there instead of our models being wrong? Is there a reason that I do not know?
Pedro

 

[thierrykauf]

Incorporating dark matter and dark energy, is a way of saying our models are (were) wrong. However, save for the common adjective, dark energy and dark matter have opposite roles. Dark energy is as dark as black body is black: not at all. Dark energy is a vacuum residual energy density. This energy density appears in Einstein's equations with the same sign as a cosmological constant. That is, it contributes to the expansion of the universe (hence the name repulsive energy). It is necessary to provoke inflation, this hyper rapid expansion of the universe that preceded the big bang. Last, dark matter, for one, is really dark. The reason is that visible matter only accounts for 10% of the density of the universe. To wrap this up: physicists will never call an equation wrong until they can replace it with a better one.

 

[Janus]

Hey everyone. This question is coming from someone who is just a science entusiast, with no training whatsoever in physics, and the enquiry comes from genuine curiosity.

To my understanding, dark matter and energy are concepts created after we made some observations that we could not account for just by looking at what we see. Example, the gravitational pull from the matter we observe shouldn't allow galaxies to be pulling away from each other, so, there must be something (dark energy) that is doing it.

No. Dark energy is not required to explain why the galaxy is expanding. Science was quite content with an expanding universe long before the concept of Dark Energy was raised. The idea of dark energy didn't come into things until we carefully measured that expansion and noted that its rate had increased over time (We expected it to decrease over time. For the time being, "dark energy" more or less is a placeholder term for what causes this.

My question is, why do we assume that this dark stuff is there instead of our models being wrong? Is there a reason for this that I do not know?
Pedro

For dark energy, a new model might prove to be the answer. For dark matter this is increasingly less and less likely to be the case. A new model of gravity would have to explain all the observations we have made. The most damning observation against a new gravity model is that of the Bullet cluster.

The bullet cluster shows the result of a collision between two galaxy clusters. Now while we can't directly see dark matter, we can detect it by how its gravity bends light, which produces a gravitational lens effect. What we see with the Bullet cluster is a region of space with no visible matter, but does produce a gravitational lens. This is exactly what you would expect to see if the dark matter had been separated away from the visible matter in the collision. This is not an observation that can be made to fit into a different model of gravity, unless that model also includes dark matter.

The case for particular dark matter has become increasingly stronger over time and the case for a modified model of gravity has become weaker.

 

[Pedro Batista]

So, dark matter must be something that has mass but doesn't interact with light which makes it invisible.

Thanks Janus, that made a lot of sense.

 

[CWatters]

Dark matter does interfere with light (it bends it) but it doesn't form stars that produce light.

 

[jerromyjon]

The case for particular dark matter has become increasingly stronger over time

And by that you mean scattered dust, as in halos surrounding many galaxies?

 

[thierrykauf]

I'm not doing research in this area and I thought I could answer your question without checking with recent papers. I was wrong. So rather than sharing outdated information on the subject, let me direct you to papers that will help you better. For the status on the search (detection) of dark matter, I recommend¹" whose abstract I quote verbatim here "Much like ordinary matter, dark matter might consist of elementary particles, and weakly interacting massive particles are one of the prime suspects. During the past decade, the sensitivity of experiments trying to directly detect them has improved by three to four orders of magnitude, but solid evidence for their existence is yet to come. We overview the recent progress in direct dark matter detection experiments and discuss future directions." As for theory, there is an over-abundance of proposals, which I will try to summarize for you in my next post.
¹ http://www.nature.com/nphys/journal/v13/n3/abs/nphys4039.html

 

[benorin]

I'm yet another physics enthusiast (my training was in math) with a follow up question: we talk about detecting gravitational lensing of light from galaxies, the light is bent by more gravity than the visible matter in the area produces; my question is detection on a different scale. Can we detect the gravity on a smaller scale of particles? Could we look at a small region and calculate the gravity do to the regular matter in it and infer the presence of dark matter particles, like a detector? (I saw on Discovery Science channel program that they are using a purified xeon detector hoping for an interaction with dark matter shielded inside a mountain)

-Ben Orin

 

[thierrykauf]

I'm yet another physics enthusiast (my training was in math) with a follow up question: we talk about detecting gravitational lensing of light from galaxies, the light is bent by more gravity than the visible matter in the area produces; my question is detection on a different scale. Can we detect the gravity on a smaller scale of particles? Could we look at a small region and calculate the gravity do to the regular matter in it and infer the presence of dark matter particles, like a detector? (I saw on Discovery Science channel program that they are using a purified xeon detector hoping for an interaction with dark matter shielded inside a mountain)

-Ben Orin

The principle of equivalence means that gravity only acts at scales that are not infinitesimal, unlike any purely local interaction. That is why detection efforts focus on large scale events. Otherwise, your idea was correct. As for the Xeon detector, results from the Lux Dark Matter experiment which consisted of 332 live days were largely negative, with only three events identified as possible candidates for dark matter.

 

[loic patry]

I'm yet another physics enthusiast (my training was in math) with a follow up question: we talk about detecting gravitational lensing of light from galaxies, the light is bent by more gravity than the visible matter in the area produces; my question is detection on a different scale. Can we detect the gravity on a smaller scale of particles? Could we look at a small region and calculate the gravity do to the regular matter in it and infer the presence of dark matter particles, like a detector? (I saw on Discovery Science channel program that they are using a purified xeon detector hoping for an interaction with dark matter shielded inside a mountain)

-Ben Orin

Just so you have some numbers in head even if they're impossible to grasp
Gravity is 10^38 times weaker than the strong force which holds the neutrons and protons to form the nuclei
So demonstrating thus "seeing" gravity at an atomic level is impossible with our current technology

 

[Drakkith]

So, dark matter must be something that has mass but doesn't interact with light which makes it invisible.

Dark matter doesn't interact through any of the known fundamental forces other than gravity, so it cannot absorb, emit, or scatter EM radiation, no. Note that this kind of behavior isn't so unprecedented. Of all the fundamental particles, most do not interact through all 4 fundamental forces. Electrons do not interact via the strong force. Neutrinos do not interact via the strong for nor the electromagnetic force. It seems almost natural to find a particle which only interacts through gravity in my opinion.

Can we detect the gravity on a smaller scale of particles? Could we look at a small region and calculate the gravity do to the regular matter in it and infer the presence of dark matter particles, like a detector? (I saw on Discovery Science channel program that they are using a purified xeon detector hoping for an interaction with dark matter shielded inside a mountain)

I don't think we could detect it by its gravitational interactions at that scale. Perhaps they were looking to see if there were any odd interactions that couldn't be explained by the standard model?

 

[jaropat]

Hey everyone. This question is coming from someone who is just a science enthusiast, with no training whatsoever in physics, and the enquiry comes from genuine curiosity.

To my understanding, dark matter and energy are concepts created after we made some observations that we could not account for just by looking at what we see. Example, the gravitational pull from the matter we observe shouldn't allow galaxies to be pulling away from each other, so, there must be something (dark energy) that is doing it.

My question is, why do we assume that this dark stuff is there instead of our models being wrong? Is there a reason that I do not know?
Pedro

I've read the book "Electric Universe". The book is for consideration only for those who are electrotechnically competent and at the same time have at least basic knowledge of astrophysics, plasma physics and nuclear physics. Unlike the author, I am a supporter of Big Bang, but I also have my own views that do not cover the current hypotheses of dark matter and dark energy.
The dark mass of the universe are forming the proton clouds, which "failed" to connect with the electrons to the hydrogen, and the electron clouds and neutron clouds. This mass is therefore dark because it does not appear electromagnetically (we can not measure). Expanding the universe causes dark energy, the repelling electrical charges of the same sign.
In the future, proton and electron clouds will overlap again, part of dark matter, protons and electrons will change to hydrogen and other molecules, and new stars formation will occur. After several oscillations of the electron cloud around the proton cloud, space expansion will stop, gravity and later collapse of the universe will take place in a small space.

 

[Drakkith]

The dark mass of the universe are forming the proton clouds, which "failed" to connect with the electrons to the hydrogen, and the electron clouds and neutron clouds. This mass is therefore dark because it does not appear electromagnetically (we can not measure). Expanding the universe causes dark energy, the repelling electrical charges of the same sign.
In the future, proton and electron clouds will overlap again, part of dark matter, protons and electrons will change to hydrogen and other molecules, and new stars formation will occur. After several oscillations of the electron cloud around the proton cloud, space expansion will stop, gravity and later collapse of the universe will take place in a small space.

Unfortunately this is quite impossible. Non-neutral electromagnetically charged particles (in this case, protons and electrons not combined into neutral atoms) react strongly with electromagnetic radiation and we would see mountains of evidence for clouds of charged particles floating out in space. Nor would such clouds have anything to do with dark energy and the expansion of the universe. Beyond that, there is no known way for clouds of like-particles to form in the first place. All particle creation events obey charge conservation, so you would need to break a fundamental conservation law on a massive scale to get these clouds from local annihilation/creation events.

Please note that we don't generally allow serious discussion of personal theories, ideas, or hypotheses. See PF Terms and Rules for more information.

 

[f todd baker]

Very few cosmologists say it, but dark matter may not be matter at all. Evidence for "it" may be a clue that we do not understand gravity as well as we think we do. General relativity is really great but not complete. Dark matter may be the 21st century luminiferous aether--look for it as we may, it just isn't there.

 

[nikkkom]

Hey everyone. This question is coming from someone who is just a science enthusiast, with no training whatsoever in physics, and the enquiry comes from genuine curiosity.

To my understanding, dark matter and energy are concepts created after we made some observations that we could not account for just by looking at what we see. Example, the gravitational pull from the matter we observe shouldn't allow galaxies to be pulling away from each other, so, there must be something (dark energy) that is doing it.

My question is, why do we assume that this dark stuff is there instead of our models being wrong? Is there a reason that I do not know?

Your question is answered in Wiki:

https://en.wikipedia.org/wiki/Dark_matter

 

[kimbyd]

Hey everyone. This question is coming from someone who is just a science enthusiast, with no training whatsoever in physics, and the enquiry comes from genuine curiosity.

To my understanding, dark matter and energy are concepts created after we made some observations that we could not account for just by looking at what we see. Example, the gravitational pull from the matter we observe shouldn't allow galaxies to be pulling away from each other, so, there must be something (dark energy) that is doing it.

My question is, why do we assume that this dark stuff is there instead of our models being wrong? Is there a reason that I do not know?
Pedro

The short answer is that when the observations didn't line up, theorists worked in many different directions to try to explain them. As more and more observations have been collected, most of those alternatives have been proven to just not fit observation at all, or to require additional assumptions that make them unreasonably complicated. For now, the only viable models really seem to be dark matter and dark energy. There are still some theorists who are working on other alternatives, such as TeVeS for explaining dark matter as a feature of gravity. But so far they haven't been able to present a convincing case.

 

[jaropat]

Non-neutral electromagnetically charged particles (in this case, protons and electrons not combined into neutral atoms) react strongly with electromagnetic radiation and we would see mountains of evidence for clouds of charged particles floating out in space.

You're right if you mean cloud of plasma. Thank you for your response.

 

[Buzz Bloom]

One aspect of dark matter has so far been absent from the discussion. I understand that the following is a bit of oversimplification.

The ratio of the density of dark matter to ordinary matter (having quarks as constituents) is sufficiently large that it is known (with a high degree of certainty) that dark matter is not made of quarks. The reason for this conclusion is related to the density of deuterium measured by astronomical observation. It is known that some deuterium is consumed as stars produce energy by nucleosynthesis. All the deuterium that still exists now was created during the primordial nucleosynthesis period (PNP) which took place during the first few minutes following the big bang. If dark matter did consist of quarks, almost all of the deuterium that exists now would have been consumed in the PNP by the synthesis of helium.

Here is one refinement detail. One current idea about what dark matter might be made of is black holes that were formed before the PNP. These primordial black holes would have been formed from quarks, but these quarks would not be able to interact with the ordinary matter quarks during PNP because they were inside the event horizon of these black holes.

 

[Buckleymanor]

No. Dark energy is not required to explain why the galaxy is expanding. Science was quite content with an expanding universe long before the concept of Dark Energy was raised. The idea of dark energy didn't come into things until we carefully measured that expansion and noted that its rate had increased over time (We expected it to decrease over time. For the time being, "dark energy" more or less is a placeholder term for what causes this.

For dark energy, a new model might prove to be the answer. For dark matter this is increasingly less and less likely to be the case. A new model of gravity would have to explain all the observations we have made. The most damning observation against a new gravity model is that of the Bullet cluster.

The bullet cluster shows the result of a collision between two galaxy clusters. Now while we can't directly see dark matter, we can detect it by how its gravity bends light, which produces a gravitational lens effect. What we see with the Bullet cluster is a region of space with no visible matter, but does produce a gravitational lens. This is exactly what you would expect to see if the dark matter had been separated away from the visible matter in the collision. This is not an observation that can be made to fit into a different model of gravity, unless that model also includes dark matter.

The case for particular dark matter has become increasingly stronger over time and the case for a modified model of gravity has become weaker.

The galaxy is not expanding .The main premise that supports dark matter is that the galaxy should expand because of the amount of observable matter in it and the gravity that matter exerts plus the speed of it's rotation is not enough to stop it flying apart.
It is the universe that is expanding carrying the galaxy and others along with it.

 

[ISamson]

Hey everyone. This question is coming from someone who is just a science enthusiast, with no training whatsoever in physics, and the enquiry comes from genuine curiosity.

To my understanding, dark matter and energy are concepts created after we made some observations that we could not account for just by looking at what we see. Example, the gravitational pull from the matter we observe shouldn't allow galaxies to be pulling away from each other, so, there must be something (dark energy) that is doing it.

My question is, why do we assume that this dark stuff is there instead of our models being wrong? Is there a reason that I do not know?
Pedro

I think our observations are wrong, because the galaxies are so far away, that our telescopes might not be able to see all the particularities.

 

[Bandersnatch]

I think our observations are wrong, because the galaxies are so far away, that our telescopes might not be able to see all the particularities.

This might sound a bit rude, so steel yourself. Keep in mind it's not an attack ad personam - I'm just trying to get you to reconsider your methodology of acquiring knowledge.
What have you seen of the measurement methods used for either dark energy or dark matter to give weight to your assertion? You state is as a fact. Have you looked at error bars in spectroscopic measurements of rotation curves, and decided they report values beyond what the instrument is capable of? Have you read Adam Reiss' (et al.) research on dark energy, for which he received the Nobel prize, and found some errors?

 

[nikkkom]

He does not state it as a fact. He said "I think". Apart from that, I agree: he does not seem to know how much work went into validating and cross-checking observational data.

 

[stoomart]

Here is one refinement detail. One current idea about what dark matter might be made of is black holes that were formed before the PNP. These primordial black holes would have been formed from quarks, but these quarks would not be able to interact with the ordinary matter quarks during PNP because they were inside the event horizon of these black holes.

My understanding is MACHOs have been ruled out as a significant dark matter candidate by gravitational microlensing observations.

https://en.wikipedia.org/wiki/Massive_compact_halo_object
https://en.wikipedia.org/wiki/Gravitational_microlensing
https://arxiv.org/abs/1607.06077

 

[Chronos]

It might be safer to say we know more about what dark matter is not than what it is. First and foremost is it is not baryonic. The most damning evidence was provided by the cosmic background radiation data of the WMAP and Planck missions - the baryonic signature was conspicuously absent in both studies. The case for dark energy is less secure, but, has improved substantially since it was first detected as a consequence of supernova studies by Perlmutter and Riess - for which they, along with Schmidt, were awarded the Nobel prize in 2011, The Dark Energy Suvey [re: https://www.darkenergysurvey.org/the-des-project/overview/] is the most promising current attempt to confirm DE. The leading suspect is the cosmological constant proposed by Einstein.

 

[Buzz Bloom]

My understanding is MACHOs have been ruled out as a significant dark matter candidate by gravitational microlensing observations.

Hi @stoomart:
Thanks for your response, particularly the third citation.

I may well be mis-remembering something I read some time ago, but I think the conclusion that MACHOs cannot be dark matter is based on an assumption about their having a large size, perhaps like those that form from supernovas. Two of the Wikipedia articles you cited listed the candidates:

brown dwarfs, red dwarfs, planets, white dwarfs, neutron stars, black holes.​

but there is certainly no reason for them to be as large as these kinds of objects. These kind of objects could not be dark matter anyway because they would be normal baruonic matter (protons and neutrons) during PNP and that would prevent the survival of the observable amount of deuterium. However, primordial black holes (PNHs) do have to be large enough so that their evaporation by Hawking radiation would not be observed, as well as being small enough so that micro-lensing effects would not be noticeable.

Wikipedia also mentioned specifically

massive compact halo objects​

which is what the acronym MACHO stands for.

https://en.wikipedia.org/wiki/Massive_compact_halo_object
A massive astrophysical compact halo object (MACHO) is any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a body composed of normal baryonic matter that emits little or no radiation and drifts through interstellar space unassociated with any planetary system.​

The the third citation is more interesting. It says:

The possibility that the dark matter is in intermediate-mass PBHs of 1 - 103M⊙ is of special interest in view of the recent detection of black-hole mergers by LIGO.​

This seems to allow for the possibility that PBHs might be dark matter.

Regards,
Buzz

 

[stoomart]

The the third citation is more interesting. It says:

The possibility that the dark matter is in intermediate-mass PBHs of 1 - 103M⊙ is of special interest in view of the recent detection of black-hole mergers by LIGO.​

This seems to allow for the possibility that PBHs might be dark matter.

The section "Lensing Constraints" on page 15 speaks directly about how little PBHs would contribute to the overall DM equation. My understanding is PBHs, if they existed, would be detected as a twinkling from microlensing, which is not observed.

 

[kimbyd]

Hi @stoomart:
Thanks for your response, particularly the third citation.

I may well be mis-remembering something I read some time ago, but I think the conclusion that MACHOs cannot be dark matter is based on an assumption about their having a large size, perhaps like those that form from supernovas. Two of the Wikipedia articles you cited listed the candidates:

brown dwarfs, red dwarfs, planets, white dwarfs, neutron stars, black holes.​

but there is certainly no reason for them to be as large as these kinds of objects. These kind of objects could not be dark matter anyway because they would be normal baruonic matter (protons and neutrons) during PNP and that would prevent the survival of the observable amount of deuterium. However, primordial black holes (PNHs) do have to be large enough so that their evaporation by Hawking radiation would not be observed, as well as being small enough so that micro-lensing effects would not be noticeable.

Wikipedia also mentioned specifically

massive compact halo objects​

which is what the acronym MACHO stands for.

https://en.wikipedia.org/wiki/Massive_compact_halo_object
A massive astrophysical compact halo object (MACHO) is any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a body composed of normal baryonic matter that emits little or no radiation and drifts through interstellar space unassociated with any planetary system.​

The the third citation is more interesting. It says:

The possibility that the dark matter is in intermediate-mass PBHs of 1 - 103M⊙ is of special interest in view of the recent detection of black-hole mergers by LIGO.​

This seems to allow for the possibility that PBHs might be dark matter.

Regards,
Buzz

Bear in mind that there are two distinct classes of MACHOs:
1. As normal matter collapses, some percentage of the collapsed matter becomes effectively invisible because it's too dim to detect.
2. Some amount of matter started out as compact objects.

Option (1) has been completely ruled out by the CMB data, which shows very clear evidence of dark matter long before any compact objects formed due to gravitational collapse. Option (2) is primordial black holes. Yes, they are a possibility. But they should still be considered an exotic possibility.

 

[kimbyd]

The section "Lensing Constraints" on page 15 speaks directly about how little PBHs would contribute to the overall DM equation. My understanding is PBHs, if they existed, would be detected as a twinkling from microlensing, which is not observed.

My understanding is that those constraints still allow PBH's within a certain mass range, which is incidentally also the mass range of the observed black hole-black hole mergers that LIGO detected.

 

[Buzz Bloom]

Option (2) is primordial black holes. Yes, they are a possibility. But they should still be considered an exotic possibility.

Hi kimbyd:

I agree that PBHs seem to be "an exotic possibility". However, at the present time, the other ideas previously discussed about what dark matter might be (e.g., neutrinos, WIMPs) seem to no longer be plausible candidates at all.

What seems particularly odd about the PBH idea, based on the https://arxiv.org/abs/1607.06077 article, is that the only sizes for PBHs that have not yet been eliminated as a dark matter possibility are those that cannot with the present technology be detected. This would require a physical mechanism for PBH creation that by an odd coincidence would create only PBHs of a size that we cannot now detect.

Regards,
Buzz

 

[PeterDonis]

Thread closed for moderation.

Edit: The thread topic has been sufficiently addressed. The thread will remain closed.

 

[Drakkith]

For clarification, this thread has been attracting some problematic posts that have now been deleted. Thread will remain locked since it is one of those topics that seems to inherently invite personal speculation.