Astronomy/cosmology questions...

[Geo]

But why isn't the equatorial bulge not of the same diameter all along it?
As far as I can tell there's a difference of hundreds of kilometers between the shortest an longest equatorial radius.

[Buster's Uncle]

Dark Matter and Energy Don't Exist: Astronomer Claims to Solve Universe's Greatest Mysteries With New Model
Astrophysical Journal article linked - something about scale invariance.

[Rusty Edge]

Dark Matter and Energy Don't Exist: Astronomer Claims to Solve Universe's Greatest Mysteries With New Model
Astrophysical Journal article linked - something about scale invariance.

I assume this is a mathematical model.

Having typed that, I wonder what else it could be... and could they get countertop expanding universe kits in the stores by Christmas.

[Lorizael]

I'll admit I don't have a strong grasp on what this guy is saying, but here's what I can piece together. In general relativity, mass-energy acts like regular old gravity in most situations. But there are times when general relativity diverges from a basic attractive force. One of those times is when the density of some amount of stuff remains constant as space expands. In those situations, general relativity actually produces a repulsive effect that we call the cosmological constant and which we observe as dark energy's accelerated expansion of the universe.

What this guy is saying is that the scale invariance of empty space--that is, the idea that empty space looks the same even as the universe expands or contracts--should similarly act as a cosmological constant and cause a dark energy-like effect. I don't know enough of the underlying physics and math to evaluate this claim. It could be true, but it could also be true that he's conflating terms and concepts that look similar.

He then goes on to say that this effect should also produce extra acceleration on galactic scales. This is where he's getting rid of dark matter. And this is where he runs into a problem. The first evidence for dark matter was the fact that stuff in the universe (at the edges of galaxies, in galactic clusters) appears to be moving more quickly than would be suggested by the mass we can see. So the dark matter hypothesis is that there is extra matter we can't see that also contributes to gravitational acceleration. But of course, an alternative hypothesis is just that gravity produces a little extra acceleration at very large scales. This is what MOND and other proposals suggest. And that's where this guy is going, too.

The problem is that we have evidence for dark matter that manifests as more than just extra acceleration. As I talked about in a previous post, if dark matter doesn't interact with regular matter, then sometimes dark matter and regular matter won't be in the same place. If we can see effects of gravity in places where this is no regular matter, that's a very big hint that we're seeing the effect of non-interacting matter--dark matter. A little bit of extra acceleration baked into your equations can't account for this.

The strongest piece of evidence for the separation between dark matter and regular matter comes from the CMB. The cosmic microwave background is an imprint of the universe from 380,000 years after the big bang. The very cool part about the CMB is that it records the acoustics of the early universe. The universe was ringing like a bell as matter contracted and expanded. Given the density, size, and composition of the universe when the CMB formed, you can predict which frequencies of contraction and expansion would have been most prominent at that moment.

But this is also where you can separate regular and dark matter. Regular matter contracts, heats up, and expands again. Dark matter, because it doesn't interact, just contracts. This means the contraction of dark matter interferes with the expansion of regular matter, which distorts the peaks and troughs of the CMB. When you look at the CMB spectrum, it perfectly matches a universe in which peaks representing matter expanding are stunted by the effects of dark matter contracting. No theory that just adds an extra bit of acceleration on large scales can account for this, because the extra gravity of dark matter is opposing the gravity of regular matter rather than adding to it.

[Buster's Uncle]

You've read the Astrophysical Journal article?
https://arxiv.org/abs/1701.03964

[Lorizael]

I read the abstract and skimmed the article. Much of the math is beyond me, at least until the general relativity bits reduce to more familiar stuff. So I can't really speak to the theoretical underpinnings. But I'll note that the ApJ paper only covers consequences for a cosmological constant. There's another paper that deals with the dark matter stuff.

[Rusty Edge]

Okay, as I've said before, black matter and black energy often sound like scientific fudge factor nonsense in an ether/humors/cold fusion sort of way- essentially secret magic that explains what they observe and don't understand.

BUT I"VE BEEN CONVINCED THAT SUCH STUFF EXISTS BECAUSE OF CROSS REFERENCED OBSERVATIONS, MEASUREMENTS, CALCULATIONS THAT FIT, etc. IT EXISTS.

So at the moment I'm wondering what it might be. A term other than "dark".  For example, could dark matter be simply something like dust made of quarks, which only has mass & effects in the aggregate? 

Or might "tangential", "extra-dimensional", or "off-spectrum" be better descriptors with fewer connotations for this energy and matter ?
















 

[Lorizael]

Dark matter has acquired negative connotations, but it's really a very apt name. Matter that doesn't interact with light--dark matter.

In terms of what it might be, there are two broad categories: MACHOs and WIMPs. The former are MAssive Compact Halo Objects and refer to objects made of normal matter that are (a) dim and (b) non-interacting. So this includes things like rogue planets and primordial black holes. Technically, these objects could interact with normal matter and light, but because they are very small the odds of them doing so are extremely slight. So if there were enough of these objects, they could act like dark matter.

MACHOs have fallen out of favor recently because of a few different measurements. One is gravitational lensing. We can actually find planets and dim stars by watching the sky for sudden changes in the brightness of background stars. If a star brightens in just the right way, the conclusion is that another object passed in front of that star and warped the light with its gravity. Because we know how much gravity there is from dark matter, we can predict how often we can see such events. We're not seeing enough, which puts strong limits on the contribution of MACHOs. The other problem is that the big bang lets us predict how much of each element should be out there in the universe, and when we look at primordial gases and stars, we see exactly what we're supposed to see. That wouldn't be the case if most matter in the universe were locked up in MACHOs.

For all that and some other reasons, this is why cosmologists favor WIMPs, which are Weakly-Interacting Massive Particles. This is where you get help from the particle physicists and ask them to dream up weird particles that fit the constraints of cosmological models. They have a bunch of possibilities that can get quite strange, some of which have been ruled out by the LHC and various dark matter detectors we have built underground.

As far as quark dust, that probably doesn't work as a suggestion. Lone quarks can't exist in nature because of something called color confinement. This is a weird concept, but basically there is a property like charge for subatomic particles, but instead of positive and negative, there are three choices. Particle physicists call them colors because why not. Charged particles attract each other, and the same is true for particles with color. The difference is that the force attracting particles with color gets stronger the farther apart they are, so quarks are only seen clumped together with other quarks because a lone quark would be pulled to the nearest other batch of quarks veeeeery strongly.

In nature, the stable varieties are pairs and triplets of quarks. The most stable triplets out there are protons, which are just regular matter. After that you get neutrons, which only last about 15 minutes on their own. Half lives go down pretty quickly after that (millionths of a second kind of thing, produced only in particle accelerators).

[Rusty Edge]

Thank you very much for that, Lorizael.

What of Dark Energy? Is it simply out of the light spectrum, or is it something weird?

[Buster's Uncle]

Also relevant-  A Mysterious Galactic Glow Hints at Hidden Pulsars

[Lorizael]

Thank you very much for that, Lorizael.

What of Dark Energy? Is it simply out of the light spectrum, or is it something weird?

Basically the only thing known about dark energy is that it acts like a cosmological constant. That is, there's some amount of mass-energy in every bit of space, and it's the same everywhere, and in general relativity that produces a repulsive effect which accelerates the expansion of the universe. As far as what it really is, that's a big mystery.

If you look at quantum physics, it says that the vacuum of space should have energy itself. And since most of space is a vacuum, that would seem to work as a candidate for dark energy. But when you try to calculate how much energy might be in the vacuum (there's not necessarily a definitively correct way to do this), it comes out to be ~120 orders of magnitude (that is, a 1 followed by 120 0s) more powerful than the observed effect of dark energy. So that's a pretty bad prediction.

[Buster's Uncle]

A question I've asked concerning relativity/cosmology, that I think is probably crucial.
What is Space? The 300-Year-Old Philosophical Battle Still Rages Today

[Rusty Edge]

A question I've asked concerning relativity/cosmology, that I think is probably crucial.
What is Space? The 300-Year-Old Philosophical Battle Still Rages Today

Buncle, I was wondering about that question myself, as I read this thread and some tangents on Wikipedia. To the extent that we don't know what dark energy is, but it acts like a cosmological constant, it's starting to sound suspiciously like "ether."

WHAT IF the key has more to do with the nature of space itself rather than theoretical dark energy? Is space elastic? Is it subject to distortion by force from things like gravity or the big bang?  I guess black holes suggest that. If so isn't it possible that stuff on the fringes of the universe is accelerating because it's entering normal, undistorted space, and our physics is based on our perspective, our world, where mass and weight seem the same at sea level, and adapted as we change altitude, get beyond our atmosphere, Earth's gravity, The Sun's etc., and yet hasn't adapted to the rules of "original space." Or maybe the Universe is something like a pond, the Big Bang a rock tossed into it, and the acceleration we see in the distance is something crossing the compression wave rings distorting the pond's surface.

Well Lorizael, I don't mean to impose on your time trying to explain the unknown. Then again, the nature of space itself is sort of the interesection of philosophy and cosmology and exactly the kind of thing you might like to talk and theorize and speculate about, but if so, do it when you have the time and inclination. 

[Lorizael]

I'm taking this out of order. There's a lot in your post.

Well Lorizael, I don't mean to impose on your time trying to explain the unknown. Then again, the nature of space itself is sort of the interesection of philosophy and cosmology and exactly the kind of thing you might like to talk and theorize and speculate about, but if so, do it when you have the time and inclination.

I'm glad for the discussion.

Is space elastic? Is it subject to distortion by force from things like gravity or the big bang?  I guess black holes suggest that.

Einstein's general theory of relativity says exactly that. I'm sure you've seen the depictions of gravity where space is a giant rubber sheet and a black hole is plopped in the middle, causing the sheet to sag downward. This is supposed to convey two ideas: (1) mass distorts space and (2) acceleration due to gravity is really just objects following the curve of space.

Getting a bit into the relationism/absolutism philosophical angle from above, relationism says there is no space, just relations between objects. Mathematically, those relations are coordinates. Like, say, I am 3 miles north and 2 miles east of you, or something like that. Naively, we think of coordinates in a Euclidean way, as happening on a flat plane where triangles and circles behave themselves.

But there are other types of geometries, which you know just by looking at world maps. You have to distort distances (coordinates) in order to get the curved Earth to lay flat. The technical meaning of this is that a curved space has a different "metric" than a flat space, where metric just means "way of figuring out how far away things are from each other."

What general relativity tells us is that mass-energy "distorts" space by changing the metric, changing how you measure distances between objects. According to general relativity, there doesn't have to be a real, physical thing called "space-time" out there containing all the stuff in the universe. You can do perfectly well just thinking about how the metric changes in response to mass.

If so isn't it possible that stuff on the fringes of the universe is accelerating because it's entering normal, undistorted space, and our physics is based on our perspective, our world, where mass and weight seem the same at sea level, and adapted as we change altitude, get beyond our atmosphere, Earth's gravity, The Sun's etc., and yet hasn't adapted to the rules of "original space."

An important thing to remember about astronomy is that we're looking back in time. We cannot see what is happening to extremely distant galaxies right now. We can only see what was happening to them millions or billions of years ago. So when we observe the accelerating expansion of the universe, we're not seeing very distant galaxies being pulled away from us at ever increasing speeds.

Instead, what we're seeing is that the expansion rate used to be slower in the past. The evidence for this comes from how much light gets stretched out on its journey to us. For relatively nearby objects, there's been a loooot of stretching. But for very, very distant objects, there's been more stretching but not as much as you would expect given how far away the objects are. That means the expansion of space is fast now. It's right here that we see the accelerated expansion.

To the extent that we don't know what dark energy is, but it acts like a cosmological constant, it's starting to sound suspiciously like "ether."

I am not convinced that aether should be a scientific boogeyman. Sometimes the aether is real. In the 19th century, Faraday postulated that invisible electric and magnetic fields emanated from all charged bodies and that these fields were responsible for the many weird electric and magnetic effects you could produce in a lab. He was right.

In the 20th century, Pauli postulated that an unbalanced equation in nuclear reactions meant there was a tiny, uncharged, non-reacting particle we could never hope to detect that made the math work. It got called the neutrino. He was right and we detected it.

Maybe dark matter and dark energy will go the way of the aether. Maybe they'll go the way of electric fields and neutrinos. I'm not sure we're in a position to judge and I don't think any supposed parallel is evidence that something is amiss in cosmology.

[Rusty Edge]

I guess I can see your point on the aether thing. Isn't the Bell labs radio telescope discovery of the background noise from The Big Bang a similar example? Something ubiquitous that eventually had no other credible alternative explanation?

Some of the other parts of your answer I may follow up on when I have a better grasp of it.