Researchers solve mystery of the galaxy with no dark matter
phys.org
phys.org
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
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.
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.
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.
People have really though this stuff through! Some of them for years!
Our sun moving away from the galactic center is also unlikely as if it had been much further in during it's creation phase, it would have been unable to create planets that support life. A galactic year is only a few million earth years, so our orbit must have remained in the stable region for the first billion years or so and we continue to be inside it. That points to a stable orbit around the galaxy.
If it was a firework wheel our current position would mean that sun would have been traveling to the outside of the galaxy far below escape velocity or would have been unable to produce life-bearing planets.
no. It is 250 million years.
>so our orbit must have remained in the stable region for the first billion years or so and we continue to be inside it.
not really. While moving on that spiral trajectory, we've made only 16 rotations since Sun's formation. When the life on our planet started our neighborhood looked very different, not the boonies we're at right now :)
I don't see how that contradicts is being a few million years?
>not really. While moving on that spiral trajectory, we've made only 16 rotations since Sun's formation. When the life on our planet started our neighborhood looked very different, not the boonies we're at right now :)
Orbits don't really do spiral patterns in practice. Atleast not at this scale. The star would either be in a fairly stable orbit or on trajectory to leave the galaxy (minus elliptical orbits but those would carry the sun in and out of the dead zone quite often).
Moving on a spiral trajectory does happen in orbital mechanics but only in one direction; inwards. Outwards spiraling would require quite a few quirks in the galactic gravity field.
The spiral pattern itself isn't the result of a firework wheel, it's quite a quirk in the distribution of stars, though there are various models that explain them, they all involve very stable orbits for most stars.
not exactly. The gravitation gathers the stuff closer, and at the same time the rotational momentum must be preserved. So initially that causes the galaxy matter into elliptical shape and from elliptical galaxy the disk forms and is thinning and spreading further to preserve the total rotational momentum of the galaxy compensating for the loss of the momentum by the matter gathering at the bulge. The radiation pressure seems to be a major component mediating that process (and in particular causes and supports the push away of the stars from the bulge&bar - part of the kinetic energy addition you mentioned).
This is why we don't "observe" that much DM in elliptical galaxies as that momentum spread by means of forming the disk is only starting there.
It's not the rotation that assists, it's the planet's linear -- well, bent, but that doesn't help -- that does it.
There is this notion of "frame dragging" in general relativity, but its effect is practically impossible to measure. They lofted a satellite to measure its effect over several years because it took that long to accumulate enough to measure.
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.
> 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.
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...
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
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).
Oops, yes. Wrote that backwards. Thanks for the correction.
If only Donald Rumsfeld had heard that, we might have stayed out of Iraq.
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.
It will take a future generation of plasma-dynamics-loving astrophysicists to change the ... er, dynamic. Presuming we live that long.
I wish they had listed the five methods, I can't think of that many ... (?!)
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.
It's all, "show me the math or GTFO!". Understandable, really.
Anyone able to explain what the new analysis was that led that existing data to be interpreted so differently?