Researchers solve mystery of the galaxy with no dark matter

https://phys.org/news/2019-06-mystery-galaxy-dark.html

by curtis • 7 years ago

64 40 7 years ago

phys.org

p1necone• 7 years ago

I want to know more about how it was measured incorrectly the first time.

pdonis3• 7 years ago

This paper is not based on new measurements. It's based on a new analysis of existing data.

thedudeabides5• 7 years ago

looks like they overshot by around 50%

Anyone able to explain what the new analysis was that led that existing data to be interpreted so differently?

sriku2• 7 years ago

A naive question (not read the actual paper or know much about the field) - if requiring dark matter to explain galactic physics can be dispensed with by a distance correction, doesn't "we've been underestimating distances all this time" serve as a simpler fact of galactic physics than needing "there is mysterious dark matter everywhere" to explain galactic physics?

idlewords2• 7 years ago

There are several different features of galaxies that dark matter explains. One of them is the speed with which stars orbit the center of a galaxy—this "rotation curve" is independent of how far the stars are from the galactic center, where if there were no dark matter, you would expect the outermost stars to go slower. That is an effect that is not affected by errors in measuring the distance to remote galaxies. We can see it in our own galaxy! https://www.e-education.psu.edu/astro801/content/l8_p8.html

In general, if you can think of an explanation that does away with the need for dark matter, you can also assume experts have thought hard about it and rejected it for good reason. Nobody likes dark matter as a hypothesis, everyone remembers analogous complications in the history of science that proved to be signposts to a better theory, and everyone knows that a Nobel prize waits for anyone who can find the correct explanation.

trhway3• 7 years ago

One of them is the speed with which stars orbit the center of a galaxy

do we really orbit it? I mean is it a stable orbit (that is the option which requires DM ) or a just a spiral trajectory (no DM required)? The spiral galaxies look like a fireworks wheel https://depositphotos.com/69366109/stock-video-fireworks-pin... , and without DM our "orbital" speed being higher than otherwise expected Keplerian orbital speed means that we're just moving away on a spiral trajectory from the center the same way like the sparks fly away on a spiral trajectory from the center of the fireworks wheel.

sriku2• 7 years ago

Good link! Thanks.

you can also assume experts have thought hard about it and rejected it for good reason.

Of course. It seemed more of a reporting anomaly to focus exclusively on the distance correction if other factors also exist that would've required dark matter.

pmoriarty• 7 years ago

"In general, if you can think of an explanation that does away with the need for dark matter, you can also assume experts have thought hard about it and rejected it for good reason."

Nobel prizes have been given for explanations which were at first rejected for what seemed like good reasons at the time, but which later proved to be the wrong reasons.

The first such example which springs to mind is the explanation of helicobacter causing ulcers.

Back to astrophysics, black holes were once considered science fictional phenomena.

privong4• 7 years ago

if requiring dark matter to explain galactic physics can be dispensed with by a distance correction, doesn't "we've been underestimating distances all this time" serve as a simpler fact of galactic physics than needing "there is mysterious dark matter everywhere" to explain galactic physics?

It would be a simpler explanation, but that would require that the distances to _all_ galaxies are overestimated. Presently there's no good evidence that our distance estimates for galaxies are systematically overestimated.

Something like this did happen in the 1940s, when it was discovered that a key type of star in the cosmic distance ladder (Cepheid variables) actually came in two types, with different period-brightness relations. Recognizing these two types and correcting distances doubled the distances inferred to nearby galaxies: https://en.wikipedia.org/wiki/Cepheid_variable#History

ajross2• 7 years ago

To be fair, the cepheid correction was presaged by good data from elsewhere in science. Until the 40's, it was possible to date the earth (via isotope ratios in decay products trapped in rocks) as older than the universe.

pdonis3• 7 years ago

that would require that the distances to _all_ galaxies are overestimated.

No, underestimated. The estimate of the distance to this particular galaxy was reduced--that's what made its observed parameters consistent with dark matter. So if it were really true that no galaxies have dark matter, our current distance estimates would all have to be too low--in the case of this particular galaxy, 13 Mpc (consistent with dark matter) vs. 20 Mpc (if there were no dark matter).

ajross2• 7 years ago

if requiring dark matter to explain galactic physics can be dispensed with by a distance correction

It's the other way around. We see dark matter everywhere except these galaxies. And the reason for that isn't that the models for dark matter were wrong, it's that our distance measurement for those few galaxies was wrong.

I mean, sure, I guess it's possible that we got the distances to the handful of galaxies right and those to, y'know, the rest of the universe wrong. But...

jvanderbot• 7 years ago

Yeah, I love this article's sense of relief that indeed, we must indeed use the strange dark matter correction factor. Whereas before, it was a perfectly normal mass of matter that we just couldn't properly measure the distance to. "Oh thank god, we don't understand this galaxy after-all".

hulahoof• 7 years ago

It returns to the known unknown of dark matter on a few galaxies than a new unknown unknown on the remainder of the universe.

pdonis3• 7 years ago

if requiring dark matter to explain galactic physics can be dispensed with by a distance correction

It's not that simple.

First, as privong pointed out, our estimates of distances to all galaxies would have to be wrong, and we have no evidence that that is the case.

Second, dark matter explains more than just galaxy rotation curves and other observed parameters. It also explains why our universe has structure at all (galaxy clusters, galaxies, and stars) even though the data on Big Bang nucleosynthesis puts a strong limit on the amount of ordinary matter (baryons) in the universe, and it's far too small to account for the amount of structure (gravitational clumping) that we see.

whatshisface• 7 years ago

If MOND was dealt a near-lethal blow with the discovery of a galaxy that had no dark matter (implying that dark matter was a thing you could put in and take away, not a universal law like MOND), now that the anomalous galaxy is gone, does that mean MOND is back?

cwmma• 7 years ago

MOND has other issues, not least of which it only explains galaxy rotational curves, it doesn't explain the larger structures of the universe (which dark matter does explain)

ncmncm5• 7 years ago

Large-scale structure has been attributed to plasma-dynamic effects. It's not a popular hypothesis, apparently in large part because plasma dynamics is hard. So plasma is generally agreed to be there (to the exclusion of everything else, really -- except, y'know, dark matter), but is not allowed to (be said to) do anything dynamical and hard to model.

It will take a future generation of plasma-dynamics-loving astrophysicists to change the ... er, dynamic. Presuming we live that long.

privong4• 7 years ago

It's worth nothing that the authors of the original NGC1052-DF4 article claiming little to no dark matter have disputed the conclusions of the Trujillo+2019 (i.e., the paper the phys.org article is based on): https://ui.adsabs.harvard.edu/abs/2019RNAAS...3b..29V/abstra...

I think it's safe to say that the jury is still out on this one.

Edit: One reason this is difficult is that you can think of galaxies as having two major contributions to the line-of sight velocity we observe from earth: expansion of the Universe ("Hubble flow") and "peculiar" velocity (motion due to the local gravitational field in which the galaxy lives). The latter velocities are typically a few hundred km/s (or later if the galaxy is in a cluster). NGC 1052 has an observed recession velocity of 1510 km/s (https://ned.ipac.caltech.edu/byname?objname=NGC%201052&hcons...). Assuming a peculiar velocity of 300 km/s, that means the peculiar velocity is on the order of 20% of the total velocity. But we don't know what component of the peculiar velocity is along the line of sight or whether it's positive or negative. So with these numbers it's "hubble flow" velocity could be from 1200 km/s to 1800 km/s. If you naive plug those two recession velocities into a cosmology model (e.g., the latest model derived from Planck observations) the distance range you get is 17.3-26.1 Mpc, or a ~40% uncertainty. So we cannot rely just on the Universe's measured expansion to get a sufficiently reliable distance estimate.

So one must use other techniques to measure distances. They each have their own advantages and disadvantages. In general most distance estimates are build on the "Cosmic distance ladder" and accumulate uncertainty as you move up the ladder: https://en.wikipedia.org/wiki/Cosmic_distance_ladder

_Codemonkeyism• 7 years ago

What I don't understand here is: You have a galaxy where mass doesn't add up. There are rivaling explanations, one being that you estimated the distance wrong, the other that there is no Dark Matter. Isn't the first one the more likely one you should investigate?

privong4• 7 years ago

There are rivaling explanations, one being that you estimated the distance wrong, the other that there is no Dark Matter. Isn't the first one the more likely one you should investigate?

If you look back at the original paper claiming the discovery of the dark matter free galaxy, they do explore several different ways of estimating the distance and argue that they're consistent and imply the more distant value. So that's something they had considered and attempted to cross-check between different distance indicators. Later came the Trujillo paper saying essentially, hey you didn't consider "X" which argues that the galaxy is closer. The authors of the original paper then published a Research Note (that I'd liked in my original post) saying that "X" doesn't agree with the other data.

So people did consider distance errors before claiming the absence of dark matter. But getting accurate galaxy distances Is Hard and there are multiple ways to do it. The original paper considered the distance estimates they could think of and they were essentially consistent. So they did investigate that first.

cobbzilla2• 7 years ago

Weren’t these all sparse elliptic galaxies? Didn't they all lack a SMB core?

Is it possible they are whisps of parts of galaxies thrown off in a galactic collision? Like two spiral arms synchronizing and flying off, then coalescing far from the dark matter, which consolidates in the merged galaxy. These degenerate galaxies might then understandably have little/no dark matter of their own.

ncmncm5• 7 years ago

That's called a "just so" story. Scientists are not gentle about those.

It's all, "show me the math or GTFO!". Understandable, really.

cobbzilla2• 7 years ago

christ it was an honest/curious question. i luv HN, really

8bitsrule• 7 years ago

Using five independent methods to estimate the distance of the object, they found that all of them coincided in one conclusion...

I wish they had listed the five methods, I can't think of that many ... (?!)

T-A• 7 years ago

https://arxiv.org/abs/1806.10141

Researchers solve mystery of the galaxy with no dark matter

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