astro123 · HackerTrans

Measurements of pretty much any time in the universe can test predictions made by models of the early universe. One of the main reasons we think there was inflation is from late time (near today) observations of matter density (see https://en.wikipedia.org/wiki/Flatness_problem).

astro123

4 anni fadiscuss

Inflation happened during the first tiniest fractions of a second post big bang. No telescope is going to make direct observations of the inflationary period so I'm not sure what you mean by this.

astro123

5 anni fadiscuss

I worked with someone (this is in astronomy) who said that papers in nature were the most likely to be wrong. They are in nature because they have a dramatic (new/unexpected) result. One good reason for a new/unexpected result is a mistake somewhere.

Edit: I'm not saying anything about this paper. I know nothing about this. Just a meta comment that, in really hard to get published in journals, there might be a bi-modality of papers. Really important and really wrong :)

astro123

5 anni fadiscuss

Small stars live a very long time. A star of the mass of our sun has a lifespan of ~10 billion years which gets you almost back to the beginning of the universe (~13 billion). This star is smaller (0.8 solar masses), so it can live even longer, so we don't need to look at distant objects to see early, low mass stars.

The other issue is that we just can't see individual stars at cosmological distances except in incredibly rare cases. See for example [1] where they discovered a single star at redshift 1 (roughly 6 billion years ago). Basically, if the star gets lensed in just the right way, it can be hugely magnified. This is a strong contender for the coolest observation that I know about. Galaxies are hard to see at redshift 1, to get a single star is crazy.

On your second question, generally people think that galaxies formed "inside-out". I.e. the inner region forms first, then the outer region. See [2]. Stars almost never collide with things (except maybe at the very center of the galaxy, but still super rare there), so survival isn't really a function of position in the galaxy.

That said, this observation is in stripe 82 (a very famous section of the SDSS. They observed it to greater depth than the rest of their area and many other surveys have since also observed it) which I'm pretty sure is away from the galactic plane (so not straight toward the center).

[1] https://iopscience.iop.org/article/10.3847/1538-4357/ab2888 [2] https://www.nasa.gov/mission_pages/WISE/multimedia/pia17554....

astro123

5 anni fadiscuss

That's a weird distinction to try to make. Metal does mean the elements, not the spectral lines. The presence of those elements is usually inferred from the spectral lines, but if another method was used we would still say "metal poor/rich".

astro123

5 anni fadiscuss

> It seems significant that the lens galaxy has little dark matter

I don't see that in the article? They centered on galaxy clusters which contain huge amounts of dark matter. DM is needed to account for lensing. There isn't enough mass in the stuff we can see to account for the lensing signal.

> How is DM supposed to cool, anyway

It can't! The way that normal matter cools is by radiating (giving off light). Dark matter doesn't radiate (if it did, we would be able to see it). So, while normal matter cools and settles to the center of the gravity well, dark matter doesn't and (as you say) forms physically larger structures.

astro123

5 anni fadiscuss

I think it was taken at a particular time. If it were the closest, venus would be closer than the sun.

Skimming the paper it looks like it was the positions in Aug 2003

astro123

5 anni fadiscuss

The early universe's expansion was not accelerating, as the early universe was not dominated by dark energy. I don't know off the top of my head exactly when that changed, but z=0.76 seems about right. So I'm pretty sure that is what that is.

The reason the dominant form of matter changes is fairly simple. As the universe expands, the amount of matter doesn't change, so the matter density goes as 1/r^3. The amount or radiation goes as 1/r^4 as there is an extra loss of energy due to redshifting (E = hc/wavelength). Dark energy though is (we think) a constant, so as the universe expands, the amount of it stays constant.

astro123

5 anni fadiscuss

There's no strong lensing there. If the background galaxies were lensed enough to have multiple images they would also be incredibly distorted.

But that does look like a small group/cluster.

Edit: Here is an example of strong lensing - see the very curved object just below the bottom bright galaxy https://viewer.legacysurvey.org/?ra=39.9717&dec=-1.5822&laye... This is https://en.wikipedia.org/wiki/Abell_370

astro123

6 anni fadiscuss

Eek! I guess I'm more OK with this now...

astro123

6 anni fadiscuss

The best suggestion I have is https://astrobites.org/

These are paper summaries, written by people in the field, where you are probably the target audience. I don't read it myself, but give it a go!

astro123

6 anni fadiscuss

Yup, by fringe I meant "On the edge of what reasonable people are working on". But maybe it's not a great term if people misunderstand.

astro123

6 anni fadiscuss

That's a fair point. I love reading computer hardware rumours, most of which are probably total garbage (and probably obviously so to anyone in the field). And in this case, whether the general public thinks the universe in MOND or LCDM really doesn't matter at all.

My real issue is when this reporting is on things where the general public's opinion does matter. Things that the general public might vote on. Economics, medicine, etc. Having seen this type of reporting in a field that I do know something about (and a field where there is no real incentive to mislead, again MOND vs LCDM, who cares), I'm a lot more distrustful of science reporting in fields I don't know much about (and where there are incentives to mislead).

If they had published the article exactly as is, giving you all the excitement, but just added a single line somewhere saying "this is new work that is up against a large body of previous work that points in the opposite direction. Let's see what happens, but its a cool idea" I'd be totally fine with it.

astro123

6 anni fadiscuss

No-one is making an argument by authority. Here is a nice popular article that outlines some of the issues with MOND https://www.forbes.com/sites/startswithabang/2018/03/06/only...

I just think that it is very hard to understand the scientific consensus (average view of people who spend a lot of time thinking about this) when all you read are popular science articles that tend to focus on the exciting/new/possibly game changing edges. I'm just here letting people know what the consensus is.

astro123

6 anni fadiscuss

The astrophysics journal (ApJ) is a really good journal. A majority of good astronomy papers are published in ApJ or in MNRAS. Big things go in Nature/Science + then there are other smaller journals.

(At least this is what I tend to find. This may just reflect my biases - US based, in the cosmology field.)

But just because something gets published doesn't mean it is right :) I think non-scientists don't know what "peer review" actually entails! First, as this was a MOND paper, it could well have been reviewed by someone who favours MOND. Second, even if the reviewer doesn't favour MOND, if the steps taken and the arguments given seem reasonable, I expect they would suggest it should be published.

I have no issue with the paper being published. It is important that theories have advocates who put forward the best argument for them. What I do take issue with is the skewed presentation in popular science. What sells is exciting and new, not slow and steady. And 99% of science is slow and steady.

astro123

6 anni fadiscuss

I only skimmed the paper past the abstract, but for what it is worth.

The MOND favouring group is a fringe of the cosmology community. The vast majority feel that there is enough evidence to rule it out.

I mostly mention this because I don't like that fact that popular science magazines (or at least their content that I see posted here!) has a bias towards "new and possibly exciting" or "controversial" research. Which I understand - revolution is more interesting that "physicist reduces error bars by 50%. Big picture unchanged". But, if all you read is these articles, you will get a very skewed idea of what the consensus is.

Just so there is no confusion, modified gravity as an explanation for Dark Energy is very possible, as an explanation for Dark Matter, the consensus is that it is ruled out.

Edit: I clarified my point below but will do it here too so everyone see it. I don't have a problem with this paper, I'm glad people are writing papers with alternate explanations to the consensus, that is how science is done. But, your conclusion from reading this article shouldn't be, "ahh damn, I guess MOND is right and LCDM is wrong" and I think that is how pop-sci articles tend to frame these things.

astro123

6 anni fadiscuss

I'm not suggesting that at all! I'm just saying it took 30 odd years to take off/break out of the nuclear physics world.

Though rereading my first comment that wasn't entirely clear...

astro123

6 anni fadiscuss

Kringing is the same thing as gaussian process regression, and astronomers use GPs a fair bit. I'm not sure whether they are used more widely.

My favourite forgotten/isolated statistical method is MCMC. These were first used by nuclear physicists at Los Alamos in the 40s/50s, but weren't really recognized more widely until the 80s. This is probably partly because only people working on bombs had access to the computing power before then, but still.

astro123

6 anni fadiscuss

I didn't know about that, that's really cool! Thanks for letting me know.

astro123

6 anni fadiscuss

I don't think that is right, though I'm not a GW expert so please tell me if I'm wrong.

I think they know the difference just by looking at the mass. i.e. we think it is hard to form black holes smaller than ~3.3ish solar masses and we don't think neutron stars can be more massive than 2.2 solar masses.

This is why we get articles like [1] where there is an issue when we think we've found something between those numbers.

And yes, the foolproof way of checking whether a NS was involved is to follow up with telescopes. But the constraints on position from GW aren't always good and so you can't always find it.

[1] https://www.sciencemag.org/news/2020/06/gravitational-waves-...

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