Questions about dark matter/energy

[imsmooth]

Does the astrophysics community really think dark energy/matter is real, or that we just haven't figured out the actual physics of what is happening and this just makes it work out (to our current understanding).

 

[PeroK]

Which would you like to discuss? Dark energy or dark matter?

 

[Orodruin]

The question doesn’t make sense. Apart from the fact that they are completely different things, we do not know exactly what they are, but we know that whatever they are they should behave in a particular way. For example, we don’t know what dark matter is - we need to find out - but whatever it is it will still be dark matter.

 

[phinds]

Does the astrophysics community really think dark energy/matter is real, or that we just haven't figured out the actual physics of what is happening and this just makes it work out (to our current understanding).

To expand on @Orodruin's answer:

"Dark Matter" and "Dark Energy" are place holder names for phenomenon which, without any possible question, exist. We know what they DO, even if we do not know HOW they do it, and whatever name you give the phenomena the eventual understanding of what they are will just change the name (maybe) but what they do is not dependent on the name, so yes, of course they exist.

 

[Ibix]

Does the astrophysics community really think dark energy/matter is real, or that we just haven't figured out the actual physics of what is happening and this just makes it work out (to our current understanding).

Dark matter and dark energy are different things.

Dark matter effects may be explicable as modified gravity or as matter that we can't see for some reason. Neither explanation is wholly satisfactory and the debate continues. "The community" does not have an opinion - it has several. Time will tell which is correct.

Dark energy, as far as I am aware, has no widely accepted physical model behind it so far. It might be the quantum mechanical energy of the vacuum (although the numbers aren't hopeful) or it might be something else. So again, I don't think there's a single opinion.

 

[ohwilleke]

Does the astrophysics community really think dark energy/matter is real, or that we just haven't figured out the actual physics of what is happening and this just makes it work out (to our current understanding).

This is the subject of active academic debate upon which a consensus has not been reached.

Dark Matter

The majority view is that dark matter is real, and a minority view is that it is a gravitational or 5th force effect of some kind, possibly due to misapplying the gravitational laws we already know and possibly due to some modification of Standard Einstein's General Relativity (GR).

The pros and cons of these alternatives are well explored in other threads in this forum.

Dark Energy

In the case of dark energy, as a practical matter, in the LambdaCDM model (which is the paradigm and baseline model used by cosmologists despite its known problems), "dark energy" phenomena are explained with a cosmological constant term in Einstein's GR equations, and this is the "baseline" assumption with respect to which alternatives are evaluated.

One of the most common things to do is for the cosmological constant to be reinterpreted in a physically equivalent way, as a very weak, constant, energy field with a fixed amount of energy per unit volume, that grows in aggregate amount as the light cone of the Big Bang expands. Using this description of the cosmological constant as a starting point, instead of a single number added to the General Relativity equations without a cosmological constant, has some desirable features.

If you use a dark energy field equivalent to the cosmological constant, instead of the cosmological constant itself, it is a lot easier to tinker with in the event that your observational data doesn't perfectly match what it should look like if the cosmological constant hypothesis is correct. The way that you tinker with it, in order to modify GR with a cosmological constant, is by having the strength of the dark energy field change over time and space according to some formula.

For example, some astronomers think that their astronomy observations would be a better fit to a dark energy field that was initially stronger than it is now, a hypothesis called "early dark energy." This is easier to fit if dark energy is actually a real, physical thing.

Thinking of the effects of the cosmological constant as a real world energy field also makes it easier to quantify the total amounts in the universe of ordinary matter other than neutrinos, neutrinos, radiation, dark matter, and dark energy in a well-defined and consistent way that can be measured with precision, and are comparable to each other in a meaningful way. The LambdaCDM model of cosmology takes this approach, with the relative proportions of different kinds of mass-energy in the universe as a whole used as parameters for the model.

Another virtue of thinking of dark energy as a real physical field is that it makes dark energy and another cosmological concept, called cosmological inflation, easier to integrate with each other, because then you just have two real physical energy fields, or perhaps even two different manifestations of a single real physical energy field.

But this isn't the only possible way to explain dark energy phenomena.

There are several quite subtle ways in which it possible to modify Einstein's GR equations that reproduce the observations that motivate and fix the value of the cosmological constant, without actually inserting a cosmological constant in the equation. Some astrophysicists are optimistic that one of these alternative GR equations might be correct because they are "more beautiful and elegant" without a cosmological constant attached. These alternatives to GR don't require energy fields of dark energy to exist.

Also, a cosmological constant can make formulating a quantum gravity analog to general relativity (which there are good suggestive reasons to think should exist) significantly harder to formulate. So, if you can formulate an alternative to GR that replicates dark energy phenomena without a real energy field and without a cosmological constant, this could be a better foundation for a quantum gravity version of that theory, than trying to come up with a quantum gravity theory that works from GR (a task which theoretical physicists have been attempting and failing at for almost a century now).

One of the big problems with coming to any consensus view on the reality of the physical energy fields that are sometimes used to explain the phenomena often attributed to "dark energy" is that our measurements of "dark energy" aren't very precise (low single digit percentage precision and these observations also have a lot of hard to quantify potential sources of systemic errors and theoretical model calculation conceptual errors). This is because, formulated as a dark energy field, this field is extremely weak to the point of being basically undetectable directly with man made detectors. The total amount of hypothetical dark energy is huge, but only because it is absolutely everywhere and there are vast amounts of virtually empty space in the universe and this adds up.

Anyway, presented with any two different possible explanations for dark energy phenomena, our observations frequently aren't good enough to say which theory is right, with a high level of statistical confidence.

Furthermore, a key physical constant closely related to dark energy, called the Hubble constant, has been measured in different ways producing inconsistent values, which suggests that the overall conceptual basis of the cosmological constant model may be flawed. This has made the search for alternatives more urgent.

 

[collinsmark]

Regarding the dark matter part of the original post, here is a lovely video by Dr. Angela Collier, explaining that "Dark Matter" is not a theory in and of itself, but rather is a collection of observations.

Sure, there are individual (and sometimes disparate) theories to explain the dark matter observations, but dark matter, at least at this time, isn't itself a theory: it's a collection of observations.

The video is worth watching at the very least (for the OP) because it introduces the list of these observations.

(And as others have pointed out in this thread, "Dark Matter" is a placeholder for a more detailed explanation/theory, once we know more about it. But for the time being, dark matter is a collection of observations.)



Dark energy, while not discussed directly in the video, is likewise, at this point in time, a collection of observations (albeit a completely different set of observations about different cosmological aspects).

[Edit: As a side note, I love Dr. Angela Collier's videos, as they are chock-full of extremely subtle bits of humor together with serious and well researched, informative sources. The effort she puts into the editing is brilliant. It's both seriously informative and somehow, also comedic gold. Be prepared to pause the video from time to time, so as not to miss the subtle bits of commentary splashed on the screen.]

 

[srb7677]

I am no expert so will likely spend a lot of time having things explained to me by those with more knowledge. I am a bit like someone with a passing interest in cosmology, who watches documentaries, and reads books and magazines on the subject, along with many other interests, like history and politics, trying to mix here with experts in the field. I feel a bit out of my depth, so will read much more than I post.

However, my simple understanding is that dark matter was hypothesized in the first instance to explain the gravitational behaviour of rotating galaxies, which only make sense, according our current understanding of gravity, if there is a lot more matter in the galaxy that we cannot see. In other words, we have inferred the existence of matter we cannot actually see by it's observed gravitational effects. Is this correct?

And I understand that some scientists are proposing a modified form of gravity to explain the observations rather than invoking dark matter. Though insofar as I can tell majority scientific opinion seems to be behind the existence if dark matter, though there is no consensus on what it might actually be yet. Is this correct?

What evidence aside from the observed gravitational effects on galaxies, is there for the existence of dark matter? And what suggestions have scientists thus far come up with as to what it might be?

And is there any evidence to support or refute a modified theory of gravity?

As an amateur in these matters, it does seem to me that the more humanity learns about the deep workings of the universe, the more we come to see how much we still don't know. And I find it awe-inspiring that learned minds can understand as much as we do already using mathematics, and the fact that the universe seems to operate on mathematically understandable principles.

 

[Ibix]

And I understand that some scientists are proposing a modified form of gravity to explain the observations rather than invoking dark matter. Though insofar as I can tell majority scientific opinion seems to be behind the existence if dark matter, though there is no consensus on what it might actually be yet. Is this correct?

Pretty much. "A" modified form of gravity is probably a stretch - there are quite a few versions, I believe.

What evidence aside from the observed gravitational effects on galaxies, is there for the existence of dark matter? And what suggestions have scientists thus far come up with as to what it might be?

As I understand it there is some evidence in the cosmic microwave background for dark matter. The spectrum of the temperature variation has a peak that makes sense if there's some form of matter that only interacts gravitationally. However, various lines of enquiry looking to directly detect dark matter, whether it is a new particle, or a lot of small black holes, and probably other things I'm unaware of, have come up empty. Hence a resurgence of interest in modified gravity.

And is there any evidence to support or refute a modified theory of gravity?

Loads. One that seems to be in the news lately is analysis of wide binary star systems. As far as I can tell, some analyses decisively rule out alternative gravity and some decisively rule out dark matter. So there's at least one thing we don't understand and it's very much not clear what the answer is. And nobody, so far as I am aware, has managed to make a modified gravity theory that leads to a cosmology that looks like what we see at really large scales.

So both dark matter and modified gravity have problems they can't yet solve. I don't think it's possible to say which is right yet.

As an amateur in these matters, it does seem to me that the more humanity learns about the deep workings of the universe, the more we come to see how much we still don't know.

Yes and no. Bear in mind that we've got far fewer assumptions in our theories than we used to have, and the failures we're discussing are in fairly extreme circumstances. Our current theories are extremely accurate for almost everything (that's one of the reasons it's so hard to find flaws).

 

[Ken G]

I think another way to frame the question of "modified gravity vs. dark matter" is instead of asking "which one is what's really happening," ask simply "which one provides the simpler way to parametrize what's happening." Here we find interesting support for modified gravity in some contexts, such as in orbits within a galaxy, where those orbits can be understood using fewer free parameters in a modified gravity picture rather than a dark matter picture. This is in keeping with the standard approach of "Occam's razor," which asks us to adopt whichever model involves the fewest unconstrained assumptions. But note that is much different than the often seen (and incorrect) way to state Occam's razor, that the simpler theory is most likely to be "right." Rightness is a very different concept in science than ease of modeling!

What this means is, there can never be a "winner" in the "rightness" vein between dark matter and modified gravity until either dark matter is actually directly detected (not modeled), or a complete theory of modified gravity is found that explains cosmology (and not just galactic orbits). Until then, we are in no position to even ask "which one is right," we can only look at the contexts that we have interest in, and ask, 'which one allows an accurate treatment with the fewest unconstrained parameters." In some situations, the answer to that is modified gravity, a state of affairs that I feel is too often overlooked.

For doubters, a classic example of what I'm talking about is Newton's laws of motion. It is well known that these laws are not universally applicable, and fail as badly in some contexts as modified gravity fails in cosmology. Yet Newton's laws are used vastly more often than the more accurate and more universal laws we already have. So much for universality vs. practicality.