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 Post subject: Re: James Web Telescope
PostPosted: 08 Aug 2022, 13:24 
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Username Protected wrote:
But it's been pointed out by, I believe, Sabine Hossenfelder et al. that to call this missing attractive force dark "matter"--implying that it's the result of something made of bosons, i.e., some sort of matter, is presently unjustified by the facts.

Gravity is caused by matter, we have extra gravity, we must have extra matter we can't see.

Therefore, dark matter.

Dark matter does explain the issue pretty nicely.

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All we really know at this point is that there appears be "something" that interacts with regular matter only gravitationally (as best we can determine). But to call this something "matter" (probably just a placeholder) may confuse the issue, at least for the layman.

The definition of "matter" is basically that is causes "gravity". So extra gravity means extra matter using what we know about gravity so far.

Using observations of gravity influence on other bodies we can see to detect ones we can't has a long history in astronomy.

For example, Neptune was discovered by analyzing the orbital perturbations of Uranus and that technique also led to theories of another planet, later to be Pluto, which was much harder to find.

More recently, you can't see a black hole, but you can see what it does to surrounding objects. Yet it is widely accepted black holes exist.

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 Post subject: Re: James Web Telescope
PostPosted: 08 Aug 2022, 13:30 
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Username Protected wrote:
...........snip..........analyzing the orbital perturbations of Uranus...........snip........


What would be the job title for this :coffee:


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 Post subject: Re: James Web Telescope
PostPosted: 08 Aug 2022, 13:40 
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Username Protected wrote:
...........snip..........analyzing the orbital perturbations of Uranus...........snip........
What would be the job title for this :coffee:

Ass-tronomer, of course.

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 Post subject: Re: James Web Telescope
PostPosted: 08 Aug 2022, 15:03 
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1. JF wrote: From cosmology measurements, there are also pretty detailed fits to the amount of mass in the universe, and that disagrees strongly with the known amount of baryonic matter (normal stuff) as measured by big bang nucleosynthesis. (if there were more baryonic matter, there would be more helium and heavier elements in the universe).

The amount of mass in the universe is computed primarily by determining the amount of mass needed to explain various phenomena--galactic cohesion, Hubble's constant, etc. No argument. But, again, it's not that the measurements require more mass per se, but rather indicate more gravity than we can explain, and since we don't know what could cause that extra gravity, for want of a better term, we call it (possibly misleadingly) dark matter.

2. MC wrote: The definition of "matter" is basically that is causes "gravity". So extra gravity means extra matter using what we know about gravity so far.

That more mass = more gravity appears indisputable, but it doesn't follow that, on a cosmological scale at least, more gravity must = more mass. (As you know, mass isn't all that creates gravity; energy does, too.)

The measurements could be wrong (granted, unlikely at the magnitudes observed).
Our understanding of gravity/spacetime, the inverse square rule, or the gravitational constant could be incomplete, or it could be something even more profound that we clearly don't understand.

But it's far from proven that what we term Dark Matter is, in fact, any sort of matter at all.


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 Post subject: Re: James Web Telescope
PostPosted: 08 Aug 2022, 17:45 
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Username Protected wrote:
But it's far from proven that what we term Dark Matter is, in fact, any sort of matter at all.

Agreed, but it is the simplest explanation, there is matter we can't see. Not everything glows like a star.

That said, our solar system is one of the few places we can study objects that don't emit light. In this case, the sun is >99% of the mass in our solar system. Kind of surprising how little the planets add up to.

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 Post subject: Re: James Web Telescope
PostPosted: 08 Aug 2022, 18:28 
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Here's an interesting article discussing voids and dark energy.
https://nautil.us/why-we-need-to-study-nothing-22553/

Because voids don’t change much through their lives, they retain a memory of the young universe. If you want to know what our universe was like billions of years ago, you can’t look into a galaxy or cluster—too much has changed. But a void? A void today is pretty much the same as a void billions of years ago. Voids are the largest and most ancient time capsules known to science.

When I say that voids are empty, I mean specifically that they are empty of matter. But that emptiness makes them full of something else: dark energy. Cosmologists aren’t sure what dark energy is; all we know is that it’s been causing the expansion of the universe to accelerate for the past 5 billion years. We believe that dark energy is somehow related to the vacuum of spacetime, a component of all the quantum fields that suffuse reality.


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 Post subject: Re: James Web Telescope
PostPosted: 08 Aug 2022, 23:35 
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There are two things going on with gravity at cosmological scales. One seems to be a property of spacetime itself, the other acts gravitationally like ordinary matter (even though it isn't ordinary matter).

The first was given the name "dark energy". "Dark" because as far as we can tell it doesn't scatter or emit any light at all. "Energy" because the gravitational effect is the same as if space had a tiny negative energy density. Dark energy is strongly supported by observations, but not yet completely sure. Its probably related to the "inflationary" field of the early universe which is likely to be measured in the next 10 years.


The other thing causing gravity is also dark - no interaction with light. Gravitationally it acts like "matter" , it clumps together the same way a bunch of pebbles might under their own gravity. For other reasons we know its not made o protons, neutrons, neutrinos, etc or any known particles. We need a name. So "Dark......" Since it acts like material stuff, astrophysicists call it "Dark matter". "Dark stuff" would work as well.

It very difficult to match current observations without dark matter and I think almost all astrophysicists / cosmologists have given up on that by now. (that wasn't' true 10 years ago, but there are a lot more observations and many different types of observations now).


It could be lots of things. Massive particles, too high enregy to see in exsting (or maybe future) experiments. Tiny black holes. Light particles that were never heated by the big bang.


Username Protected wrote: The definition of "matter" is basically that is causes "gravity". So extra gravity means extra matter using what we know about gravity so far.

That more mass = more gravity appears indisputable, but it doesn't follow that, on a cosmological scale at least, more gravity must = more mass. (As you know, mass isn't all that creates gravity; energy does, too.)

The measurements could be wrong (granted, unlikely at the magnitudes observed).
Our understanding of gravity/spacetime, the inverse square rule, or the gravitational constant could be incomplete, or it could be something even more profound that we clearly don't understand.

But it's far from proven that what we term Dark Matter is, in fact, any sort of matter at all.


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 Post subject: Re: James Web Telescope
PostPosted: 10 Aug 2022, 12:27 
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At some point, we will need to start a new thread for the Giant Magellan telescope, but it is probably too early right now. (I think someone mentioned this early on in this thread).

The GMT will be land based (in Chile) and will have 10 times the resolution of the JWT. This is made possible by the huge size of the mirrors, since it doesn't have to fit inside a rocket ship.

https://petapixel.com/2022/08/09/1b-gia ... ames-webb/


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 Post subject: Re: James Web Telescope
PostPosted: 10 Aug 2022, 14:22 
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Yes GMT will be amazing.

There are all sorts of tradeoffs between space and ground based telescopes. In space there i no atmosphere in the way so you can see a wider range of wavelengths an in particular look at very tiny spectral features (like exo-planet atmospheres). But for the same $$ you can build a far larger telescope on the earth.

They typically target different science.

Username Protected wrote:
At some point, we will need to start a new thread for the Giant Magellan telescope, but it is probably too early right now. (I think someone mentioned this early on in this thread).

The GMT will be land based (in Chile) and will have 10 times the resolution of the JWT. This is made possible by the huge size of the mirrors, since it doesn't have to fit inside a rocket ship.

https://petapixel.com/2022/08/09/1b-gia ... ames-webb/


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 Post subject: Re: James Web Telescope
PostPosted: 10 Aug 2022, 14:33 
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Yes GMT will be amazing.

There are all sorts of tradeoffs between space and ground based telescopes. In space there i no atmosphere in the way so you can see a wider range of wavelengths an in particular look at very tiny spectral features (like exo-planet atmospheres). But for the same $$ you can build a far larger telescope on the earth.

They typically target different science.

I guess that was the point of the closing statement in the article:

Quote:
“This unprecedented angular resolution, combined with revolutionary spectrographs and high contrast cameras, will work in direct synergy with JWST to empower new scientific discoveries,” the GMTO says.


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 Post subject: Re: James Web Telescope
PostPosted: 10 Aug 2022, 15:38 
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Just think of the great planning and execution that started over 80 years ago in order to create the ozone hole so that the ground based GMT wouldn’t have to look through quite as much atmosphere.

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 Post subject: Re: James Web Telescope
PostPosted: 10 Aug 2022, 16:39 
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Just think of the great planning and execution that started over 80 years ago in order to create the ozone hole so that the ground based GMT wouldn’t have to look through quite as much atmosphere.

The ozone layer is 3mm thick. I don't think we've reduced the thickness of the atmosphere to any substantial degree. Arguably with the addition of carbon dioxide it should be thicker.

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 Post subject: Re: James Web Telescope
PostPosted: 10 Aug 2022, 16:43 
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I've gotta start using green font more.

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 Post subject: Re: James Web Telescope
PostPosted: 18 Aug 2022, 16:31 
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[youtube]https://youtu.be/piMfL12RzoA[/youtube]

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 Post subject: Re: James Web Telescope
PostPosted: 12 Sep 2022, 08:35 
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[youtube]https://youtu.be/piMfL12RzoA[/youtube]

Excellent, Doug. Thanks

Can you please point us to a site that offers high quality Webb images? I’ve seen some stunningly beautiful shots, but I’d prefer near-originals instead of screenshots of screenshots.

There must be a master site somewhere. Thanks for the help.

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