Hacker News Reader

Comment by dack | Hacker News Reader

I can't tell how certain we are of its size - it's so big that it makes me wonder whether the easiest explanation is that our methods were incorrect.

For example, if we are measuring how light bends - how do we know there aren't many hidden (i.e. not large or bright enough to be noticed from earth) objects causing additional distortion and therefore throwing off our measurements? Do we also observe it over time to get more confidence, or have some other cross-checks that give us more certainty?

Either way, every once in a while I find it wild just how much we can deduce about the universe from our tiny fixed vantage point.

View on HN

The 30 billion times the size of the sun is BBC making a mistake. It is 30 billion solar masses. Direct from durham [1] from the arxiv article [2].

techwiz137 pointed out[3] an existing known black hole, TON618, is 66 billion solar masses[4]. So the new find is large but not the largest ever found.

[1]https://www.durham.ac.uk/news-events/latest-news/2023/03/lig... [2] https://arxiv.org/abs/2303.15514 [3] https://news.ycombinator.com/item?id=35360780 [4] https://en.wikipedia.org/wiki/TON_618

Astronomy is afforded very few epistemological amenities. In a sense, N can only go up if we get independent verification from different techniques. However, our tools have become incredibly sophisticated and sensitive so we can be very sure about what we're seeing, if not what it means.

I'd imagine one of the techniques used is to look for time-varying influences in the data, which implies there are bodies orbiting each other. If there aren't any, then you can look at the spectroscopic signature of the light and see if it makes sense for a black hole. I'd imagine black holes do nothing to refracted light, but dark bodies would result in slightly blueshifted light as 'newer' light reflects off its surface. This might be what PyAutoLens does (as well as fitting the shape of distortions and such)

Even in astronomy, when one starts to say "this black hole is about the size of our galaxy", it should raise epistemological questions.

This finding seems to have a highly complex relationship to the data, and to not have been replicated anywhere yet. This is the perfect place to add a "something that looks like" before the "galaxy sized black hole".

But well, this is a criticism of journalism anyway, not of astronomy.

If a photon is deflected by a black hole, it must also impart momentum to the black hole. But this means it imparts energy as well, so shouldn't it be red shifted as well?

That's true but it's also true of some other dark body of the same mass. "do nothing to refracted light" was poor wording, my bad - it's more like gravitational lensing is the only thing that happens to its light, as opposed to adding new peaks to the spectrograph and such.

it is much more likely that our understanding of the early universe is imperfect, than it is that multiple gravity wells of that strength line up perfectly. we know our tools pretty well; well enough to have confidence that what is observed is actually representative of what is actually there.

the confusion here is because if this formed in 13 billion years, our understanding of the formation of the universe is very flawed, and generally, scientists both do and do not like it when stuff like this is proven wrong with new evidence.

they like it because discovery and increased understanding is the whole reason they became scientists in a lot of cases.

they don't like it because if this is wrong, what else is wrong, and what does that mean for everything else we think we know? what gets upended because of this?

Nothing gets upended, you just say "dark matter" and call it a day.

> than it is that multiple gravity wells of that strength line up perfectly.

This kind of alignment happens all the time. Stars that line up perfectly but are separated by light years, galaxies that line up perfectly etc.

Don't know the answer to your questions but we are not at a fixed vantage point

Fixed relative to other galaxies...

we move on Earth as the planet rotates, Earth moves within its orbit in the Solar system, our Solar system moves within its orbit in the galaxy, and our galaxy moves (we believe) within the universe, though I don't think we know how fast or what direction; generally, everything outside our local group is moving away, though.

Our sun moves around the galaxy at about 500,000 mph...and thus so do we.

The things aren't moving apart.

Space itself is expanding.

space is expanding AND things are moving apart.

where does red shift come from if things aren't moving apart?

Yes, all this movement occurs and is infinitely small compared to the distance to other galaxies.

In a cosmological scale, we are goldfish in a bowl (Earth). We can move around inside our bowl, but we cannot move the bowl itself, and if we leave the bowl we won't last very long.

Thus, our vantage point is fixed.

The bowl (and the table, for that matter) move enough for useful measurements to be made wrt things inside the milky way.

https://en.wikipedia.org/wiki/Parallax_in_astronomy

I learned now at least that our parallax measurements have been improved but they are still limited to tens of thousands of light years, i.e not reaching out of the galaxy, see https://en.wikipedia.org/wiki/Stellar_parallax#Space_astrome...

Reply on news.ycombinator.com