Conceptually, it makes sense to me that there is roughly 5 times more dark matter than regular matter. Gravity needed to hold galaxies together requires that much more matter. So while I couldn’t begin to tell you how to calculate it, it makes logical sense that we could know how much.
Dark energy, which is responsible for the expansion of the universe, seems (to me) to be a totally different concept. Like apples and oranges. What does the 68% even refer to?
You could know (get constraints on) how much dark energy there must be by subtraction. If your universe is expanding according to the Friedmann equation and you can figure out the expansion rate, the total density, and how much of the density parameter comes from matter and radiation, then you can conclude how much must be left over.
There are all kind of experiments to measure the cosmic microwave background anisotropy, observe supernovae, matter clustering, etc. to get constraints on the values of the Hubble parameter.
Dark energy is an appalling name, and just confuses everyone. It does indeed have nothing to do with dark matter.
Calculate the total equivalent energy of the universe:
we get a very big number, and one where dark energy is 68% of the total.
There are only two data points that give us an accelerating universe, and they disagree. Very distant supernovae were the first, and characteristics of the CMB are the second. Both give us rates of expansion at much earlier times in our universe, and both are lower than we observe now. So the simplest answer is that the expansion is accelerating. If the rate of acceleration measured by both these observations were the same it would have been a near slam dunk confirmation. But they don’t. So the puzzle remains.
Whilst there is general enthusiasm for dark matter, it is far from a done deal. Personally, as a non-astronomer with no special expertise in the area at all, I remain uneasy. There have been efforts to close the gap between supernovae and CMB numbers, getting both to higher accuracy, but the gap has not really improved. IMHO there is more to this than we know yet. There are murmurings from the peanut gallery that dark energy may yet prove to be an illusion. Personally I’m with them, but I’m just some shmuck on the Internet. IMHO the rush to invoke new physics has been very hasty, and is somewhat emblematic of the current problems in science.
IANAPhysicist but isn’t it the other way around? If dark energy is a real phenomenon we can’t explain yet, no “new physics” is needed - just a source for the extra energy. It is the OTHER explanation for our observations - say, that the inverse square law isn’t right at extreme intergalactic distances - that would require new physics.
Dark energy is absolutely new physics. We are modifying the nature of space-time. I’m not suggesting other new physics is the answer. I’m suggesting that any attempt to definitely explain the observations is premature. Discussions about dark energy are fine, but the manner is which the public has been informed that there is this new mysterious energy, with continual popular science exposés, is well past the bounds of reasonableness.
Might I ask where this number comes from?
So are you contending that the expansion of the universe ISN’T speeding up rather than slowing down? I know that the two measurements of how quickly that acceleration is growing disagree (the so-called Crisis in Cosmology), but I’m not aware of any theories or models that claim it isn’t happening at all. And while the two numbers do disagree on the exact rate of acceleration, it is clear that it IS happening.
Given an expanding and accelerating universe, there are basically two possible options:
Yes, it is true that any explanation of option 1 would involve some new phenomenon that is adding energy into the universe in a way we don’t understand yet. But if your options are 1 and 2, it seems extremely odd to me to describe 1 as the option that relies on “new physics” when it is option 2 that would throw our understanding of physics on its head.
For an example of what it would take to explain the observed phenomenon which we call “dark energy” using option 2, you can look at the 2018 gravitational wave study. In short, the theory was that gravity propagates through more than our 3 spatial dimensions; and that, at extreme distances (for example, between galaxies) the energy that is lost to those extra dimensions causes gravity to weaken. In other words, there is no “dark energy”, but gravity decays ever so slightly faster than the inverse square law would lead you to believe as its energy dissipates in more than 3 dimensions. So the galaxies we think are being accelerated away by dark energy are in fact simply not being pulled on by gravity with as much force as we would have thought.
In 2018 they tried to find evidence that gravity dissipates in this way by studying gravitational waves formed by the collision of two neutron stars. The hope was that if gravity really does decay in this way, we could measure the light given off by the colliding stars, determine their mass, and compare the measured gravitational wave readings to the calculated ones based on the light given off by the stars; if this theory was true, the gravity wave would be weaker than expected. But it wasn’t, and given the incredible distances involved, this confirms that gravity does not decay faster than expected to a very high degree of accuracy.
“Our formulas for calculating the force of gravity are completely incorrect with very large distances, much as how Newton’s formulas were incorrect with very high velocities” sounds much more like new physics to me than “we don’t understand where this energy is coming from YET, so we call it dark energy” - YMMV.
No. I’m saying that dark energy is too hasty to proclaim to the rafters that we have it solved. And that is what is happening. Even listing the two options we have as the definitive list is premature. Both involve new physics. Both are actually saying that our understanding of the nature of space-time is incomplete, but in different ways. Dark energy adds another term to GR. That isn’t a trivial thing, and IMHO is exactly the same as saying “our understanding of gravity breaks down at extreme scales”. It is however the most economical way of modifying GR. But it invokes the need for a new source of energy as well. So it isn’t all roses. I tend to feel that anyone suggesting modifying GR at all is being premature. Unless it makes a testable prediction.
Again, dark energy might be right. But we don’t know well enough yet, and telling every layman that we have a definitive understanding of the physics and that dark energy is a thing is just misleading.
Sure the gravity wave experiment discounted extra dimensions (much to the horror of the string theorists). So one idea for a different explanation vanished. Thankfully. That one involved more new physics than I could stomach. So that one didn’t come up with a countering explanation, but I would be very hesitant to use that as evidence that no others will succeed. Nor would I discount explanations for the observations that invoke no new physics at all. It remains IMHO just too early. So, yes, I’m not wholly convinced the expansion rate is real.
Actually, those two are perfectly consistent with each other, and there are other independent experiments that are consistent with the intersection of both. The data basically form multiple long skinny ellipses, that all intersect in a small region. And that point of intersection is also consistent with some values that we expect from theory. All of the evidence, in other words, is consistent with that 68% figure, to a high degree of both precision and confidence.
Which still leaves us with no clue whatsoever what the heck that “dark energy” stuff that we seem to have so much of actually is. Worse than no clue, actually: We do have an inkling of an idea that vacuum energy, if nonzero, would behave in a way qualitatively similar to dark energy… except that our best estimates of the strength we would expect of vacuum energy are over a hundred orders of magnitude too large. Being off by over a hundred orders of magnitude, well, that’s bad.
I’m very much enjoying this thread. But, (and I feel I’m not alone here) I would benefit from some dumbing down and explanation of acronyms. I know GR=general relativity. I think CMB= Cosmic Microwave Background but I’m not sure. Fascinating stuff!
Thanks for all the excellent answers and discussion. My takeaways, and please correct me if I’m wrong, are that with respect to the original question, dark energy is related to dark matter in the sense that you couldn’t know how much dark energy there is without knowing how much dark matter there is. And you can’t know how much dark matter there is without knowing how much regular matter there is. And we can’t know any of these without knowing how gravity works at short and long distances. Also, based on what we believe now, dark energy will continue to become a larger and larger part of the total until it is almost 100%. The ratio of dark to regular matter won’t change.
Another question that comes to mind is, are we able to trace this ratio back to the early universe, so that we know it for each point in time? IOW, do we know the rate of acceleration of the universe’s expansion at all points in time?
How much dark energy there is and how much matter there is are mostly independently determined. The matter, you pretty much do measure the amount of “normal” matter and subtract that off the total to get the dark matter.
We think we know this, and have no strong reason to believe we’re wrong.
This is correct.
Well, approximately, at least. But that’s based partly on models that we aren’t entirely sure about, so there’s considerably more uncertainty than there is for the present numbers.