Some very cool stuff from astronomers etc

[JeanTate]

In a very active thread here in this board, a couple of astronomy/astrophysics/cosmology topics have come up which many regular readers may not have heard of.

First, expansion of the universe. Yeah, the results are pretty clear: if General Relativity (GR), then the universe seems to be expanding. Expansion which is accelerating.

But it's all very indirect ... wouldn't it be cool to measure the line-of-sight speed of a galaxy, and next year repeat the measurement to see if that speed has increased?

A completely hopeless task you might say, the expected change in speed is utterly miniscule, how could you possibly measure it?

This Is How Astronomers Will Finally Measure The Universe's Expansion Directly, by Ethan Siegel explains how (maybe). Hat tip to Reality Check.

In short, a humongous telescope plus astonishing technical feats, and examining the spectra of a lot of distant quasars, over two+ decades might give a robust answer!

For more detail, check out this 2008 paper (link to arXiv abstract) by Liske et al. Cosmic dynamics in the era of Extremely Large Telescopes. The key technology is "laser frequency combs" check out this 2007 paper (link to arXiv abstract) by Murphy et al. for details: High-precision wavelength calibration of astronomical spectrographs with laser frequency combs.

Next: the cosmic web and filaments.

 

[JeanTate]

In that same thread, devoted to a strange idea about quantum mechanics, and in one of the many on the Electric Universe (Sun, Comet, whatever), the topic of the cosmic web - a.k.a. filamentary structure of the universe on very large scales - came up.

Here is a very cool image:

If you don't know what this is, and you don't bother to find out, you could easily jump to conclusions that are ... wrong. A fave seems to be "plasma!" and "electricity!"; a certainty is that it's an actual astronomical observation (or a synthesis of many such).

Yet it's from the 2005 Millennium Simulation (link to arXiv abstract). As one ISF member summed it up:

Indeed. He got basically everything about that wrong. It wasn't an observation, it was a simulation. And gravity, not electrical forces, is what produced those filamentary structures. Furthermore, what's being plotted is dark matter densities, and the EU folks generally don't believe dark matter exists. So to use this simulation (not observation) of dark matter acting under the force of gravity in order to justify EU ideas is just the height of irony.

Amazing isn't it? Gravity, and gravity alone, can produce such nice filaments etc. Why "alone"? Because dark matter interacts solely via gravity (at least in simulations like this).

Of course, the universe we observe isn't exactly like this simulation, but the large scale structures we do observe are very similar (per detailed statistical tests).

While I'm on this topic, Sabine Hossenfelder, of the BackReAction blog, recently wrote about dark matter (again); it's quite interesting: Dark matter nightmare: What if we are just using the wrong equations?

 

[Reality Check]

While I'm on this topic, Sabine Hossenfelder, of the BackReAction blog, recently wrote about dark matter (again); it's quite interesting: Dark matter nightmare: What if we are just using the wrong equations?

Sabine Hossenfelder is right. Much of the evidence for dark matter comes from GR. Analysis of the CMB using standard cosmologic models. Gravitational lensing of colliding galaxy clusters to see the separation of dark matter from normal matter. So if we are incorrectly modeling the non-linear nature of GR, this evidence could be wrong. Dark matter may be less, may be more or might not be there al all.

However "Galaxies rotate faster than expected" is GR in its Newtonian limit, not full GR. "Galaxies in clusters move faster than they should" is the classical virial theorem thus Newtonian gravity. The non-linear nature of GR is not an issue for this evidence. These results agree with the GR-based result and suggest that the models are correctly handling GR for dark matter.

The evidence for dark energy is based on GR and so affected by the modeling the non-linear nature of GR.

 

[JeanTate]

Sabine Hossenfelder is right. Much of the evidence for dark matter comes from GR. Analysis of the CMB using standard cosmologic models. Gravitational lensing of colliding galaxy clusters to see the separation of dark matter from normal matter. So if we are incorrectly modeling the non-linear nature of GR, this evidence could be wrong. Dark matter may be less, may be more or might not be there al all.

However "Galaxies rotate faster than expected" is GR in its Newtonian limit, not full GR. "Galaxies in clusters move faster than they should" is the classical virial theorem thus Newtonian gravity. The non-linear nature of GR is not an issue for this evidence. These results agree with the GR-based result and suggest that the models are correctly handling GR for dark matter.

The evidence for dark energy is based on GR and so affected by the modeling the non-linear nature of GR.

In the comments on this blogpost, you'll find some discussion of ideas and papers by Cooperstock, on using GR to model the mass distribution of spiral galaxies and whether such a model is consistent with "rotation curves" without the need for CDM (cold dark matter).

While that particular approach fails in several ways, I think it would be something worth pursuing (sadly Cooperstock is no longer with us). As couple of the commenters expressed an on-going interest in doing just that, I'm somewhat hopeful that it will be explored further.

The "galaxies in clusters move faster than they should" is, I think, the big one ... no matter what alternative to CDM (as a form of 'dark mass') is proposed, they all fail badly for this particular set of observations (or are not mentioned/tested at all).

 

[Reality Check]

In the comments on this blogpost, you'll find some discussion of ideas and papers by Cooperstock, on using GR to model the mass distribution of spiral galaxies and whether such a model is consistent with "rotation curves" without the need for CDM (cold dark matter).

While that particular approach fails in several ways, I think it would be something worth pursuing (sadly Cooperstock is no longer with us). As couple of the commenters expressed an on-going interest in doing just that, I'm somewhat hopeful that it will be explored further.

Trying to model galaxy rotation curves using GR seems to fall into the trap that Sabine Hossenfelder is writing about - how can we know that we are treating the non-linearity of GR correctly? What astronomers have generally said is that gravity is weak enough in galaxies to use Newtonian gravitation because that is what we see in the Solar System. So we have many decades of research into measuring and modeling galaxy rotation curves with the 1970's evidence of dark matter.
30 years afterward, a couple of groups of astronomers think that the orbits of stars in galaxies are different enough from planetary orbits that GR needs to be used. The stars are better modeled as a self-gravitating dust where non-linear effects are significant. So we get General Relativity Resolves Galactic Rotation Without Exotic Dark Matter (2005). There are other papers by F.I. Cooperstock & S. Tieu on the same model.

Tested by Comment on ``General Relativity Resolves Galactic Rotation Without Exotic Dark Matter'' by F.I. Cooperstock & S. Tieu (2006) which shows that it "singularly fails to reproduce the observed local mass density and the vertical density profile of the Milky Way".

This paper looked at GR vs Newtonian limit in dust models and found they agreed "thus supporting the conventional use of Newtonian physics to analyze galactic rotation curves". On claims that general relativity differs from Newtonian physics for self-gravitating dusts in the low velocity, weak field limit (2015)