I'm a cosmologist ask me anything

[grazorblade]

Oh on brian cox he is a particle physics, mostly works on experimental physics, but has a few theory papers from when he was younger by the looks of it

INSPIRE

inspirehep.net

 

[HappyGuus]

yeah I think if aliens existed that wouldn't impact our significance -to make an analogy, the first Australians are just as significant before and after the first europeans make contact and I'd view us as just as significant if aliens appeared tomorrow

Yep agree with that. Gotta look at it from both angles. From a human/indigenous side, significant. From an alien/colonist perspective, they are/were insignificant. Let's hope for some nice aliens who think we are significant and we're not their pigs growing organs for them. I'm sure some pigs think they're significant, too.

 

[grazorblade]

So it is unlikely any objects we encounter have travelled from other galaxies?

almost impossible, everything in the galaxy has a speed below the escape velocity of the milky way (about 500km per second). You would need an alien from another galaxy to give the media a bigger kick

 

[RIMB]

Have we found/identified basically all elements that are available to find in our galaxy or is there a chance of seriously different things still within our galaxy to be found? I mean physical elements we can currently see/feel/measure as opposed to what we can't yet conceive of. The lighter than air sort of solids that sci fi might write about as an example - or have we basically discovered all our galaxy's main lego parts?

almost impossible, everything in the galaxy has a speed below the escape velocity of the milky way (about 500km per second). You would need an alien from another galaxy to give the media a bigger kick

I am thinking about an object coming into our galaxy from outside. Perhaps if a smaller bang not too far away created debris heading towards us instead of all coming from our big bang.

That's my last contribution on this subject for tonight anyway.

 

[grazorblade]

Have we found/identified basically all elements that are available to find in our galaxy or is there a chance of seriously different things still within our galaxy to be found? I mean physical elements we can currently see/feel/measure as opposed to what we can't yet conceive of. The lighter than air sort of solids that sci fi might write about as an example - or have we basically discovered all our galaxy's main lego parts?


I am thinking about an object coming into our galaxy from outside. Perhaps if a smaller bang not too far away created debris heading towards us instead of all coming from our big bang.

That's my last contribution on this subject for tonight anyway.

well the elements can be listed by number of protons in the nucleus. We can synthetically make any element we want, so have discovered some that are not naturally occuring. Once you have too many protons in the nucleus the element becomes unstable and the life time is very short. It is unlikely there are any interesting stable useful elements undiscovered

No process I know of would be able to give meteors enough energy to escape a galaxy - if they collide they lose energy!

 

[grazorblade]

Yep agree with that. Gotta look at it from both angles. From a human/indigenous side, significant. From an alien/colonist perspective, they are/were insignificant. Let's hope for some nice aliens who think we are significant and we're not their pigs growing organs for them. I'm sure some pigs think they're significant, too.

I suppose to drive the analogy further, if aliens did what europeans did in mistaking first nations as insignificant that would be their error....not that we could do anything about it

 

[RIMB]

By the laws of physics would a pin head of antimatter cancel out a pin head of normal matter? Would the energy generated be large (size of orange, size of car, size of football field,...)? Can we create and contain antimatter?

Is the spread of dark matter throughout the observed universe relatively uniform or limited to specific 'locations' in space? Since dark matter is invisible to us, any chance that dark matter is 'cloaked' planets/space cities/aliens? Idle thoughts only - I would expect not as surely they would produce visibly measurable emissions/pollutants/reactions/something that would show them up - but then again a closed system would not be beyond the imagination of an alien race that could harness invisibility.

With the concept of aliens, if they originated from a non atmospheric planet they would require much less energy to escape their planet (just beating gravity is my guess) than we do.

If sound or light waves were sucked into the orbit of a very, very small but super dense something would they be invisible (no light escaping) and generally produce the theorised characteristics of dark matter?

 

[Keeper66]

We also don't know if there is anything outside the Universe or if it has a boundary

This is the bit that I can't get my head around. Logically, if the universe is expanding it can't be infinite (or can it, can you get larger than infinite?), so my brain says there must be some sort of "space" outside it for it to expand into. If there is nothing outside it, then I also can't comprehend what nothing is!

 

[Muz]

Thoughts on multiverses or cyclic universes given the 'fine tuning' that is present in our universe for it to exist. It sort of defies explanation that the one and only universe that we live in was 'exactly right' the first time a universe existed.

Yes I realise we can only exist because the conditions are just so because here we are BUT if any one of the following were out by even a tiny bit we, the planets and the stars wouldn't have formed.

I grabbed the following from chatGPT because I couldn't remember them all.

Anyone of these out by a smidge and it was curtains. Seems to lend credence that this can't be the only universe that ever was or ever will be.


Yes! Our existence in the universe depends on a delicate balance of fundamental physical laws and constants. Here are a few key factors that had to be "just right" for matter—and ultimately life—to exist:

1. The Strength of the Strong Nuclear Force
This force binds protons and neutrons together in atomic nuclei.

If it were slightly weaker, atomic nuclei wouldn’t hold together, preventing the formation of elements beyond hydrogen.

If it were slightly stronger, nuclear reactions would have burned through hydrogen too quickly, leaving no fuel for long-lived stars.

2. The Strength of the Electromagnetic Force
Determines how electrons orbit atomic nuclei and how atoms form molecules.

If it were stronger, electrons would be held too tightly, preventing complex chemistry.

If weaker, atoms wouldn’t hold together properly.

3. The Ratio of Electrons to Protons
This must be balanced so that the universe is electrically neutral overall.

Even a tiny imbalance could cause repulsive forces to overwhelm gravitational attraction, preventing galaxy formation.

4. The Strength of Gravity
If gravity were slightly stronger, stars would burn out quickly, leaving no time for life to develop.

If it were weaker, stars and galaxies might never have formed at all.

5. The Cosmological Constant (Dark Energy)
Governs the expansion rate of the universe.

If it were too large, the universe would expand too rapidly for galaxies and stars to form.

If too small, the universe might have collapsed back on itself.

6. Matter-Antimatter Asymmetry
At the beginning of the universe, nearly equal amounts of matter and antimatter were created.

If they had been exactly equal, they would have annihilated each other, leaving no matter behind.

A tiny imbalance (about 1 extra matter particle for every billion pairs of matter and antimatter) allowed some matter to survive.

7. The Mass of the Higgs Boson
Determines the masses of fundamental particles.

If it were much different, the universe could have been too unstable or too different in composition to allow complex chemistry.

8. The Right Mix of Elements (Nucleosynthesis in Stars)
Early in the universe, only hydrogen and helium existed.

Heavy elements (carbon, oxygen, iron, etc.) were formed in stars and spread through supernova explosions.

Without this process, planets and life-essential elements wouldn’t exist.

 

[Muz]

This is the bit that I can't get my head around. Logically, if the universe is expanding it can't be infinite (or can it, can you get larger than infinite?), so my brain says there must be some sort of "space" outside it for it to expand into. If there is nothing outside it, then I also can't comprehend what nothing is!

Hahahaha, feel your pain. There are heaps of different 'infinities'. Some bigger than others. For example there are more real numbers between 0 and 1 then there are integers between 1 and infinity.

Best not to think about it too hard or your head will explode.

 

[Muz]

Going slightly off track here but this is excellent.

 

[Lupi]

I've heard Muz is a big fan of Karl Kruszelnicki

 

[Muz]

Was only a matter of time before the flog arrived to pollute another thread.



Again, I'm flattered, but no thanks.

 

[grazorblade]

By the laws of physics would a pin head of antimatter cancel out a pin head of normal matter? Would the energy generated be large (size of orange, size of car, size of football field,...)? Can we create and contain antimatter?

Is the spread of dark matter throughout the observed universe relatively uniform or limited to specific 'locations' in space? Since dark matter is invisible to us, any chance that dark matter is 'cloaked' planets/space cities/aliens? Idle thoughts only - I would expect not as surely they would produce visibly measurable emissions/pollutants/reactions/something that would show them up - but then again a closed system would not be beyond the imagination of an alien race that could harness invisibility.

With the concept of aliens, if they originated from a non atmospheric planet they would require much less energy to escape their planet (just beating gravity is my guess) than we do.

If sound or light waves were sucked into the orbit of a very, very small but super dense something would they be invisible (no light escaping) and generally produce the theorised characteristics of dark matter?

yes anti matter will annihilate matter. No one has figured out how to produce enough of it to make it worth containing, but there is no principle against it, so maybe in the future. The amount of energy is even more dramatic than a nuclear bomb. For a nuclear reaction a small amount of mass is converted to energy, for matter-anti matter annihilation 100% gets converted to energy

dark matter forms "halos" around galaxies and nearly all of it is in galaxies. There is a bit of a mystery because sometimes it clumps in the middle of galaxies more than you expect and sometimes it is more sparse than you expect in the middle of galaxies. It is also a bit of a debate whether this is a mystery

Dark matter could indeed be like the visible sector, right up to the point where there are dark stars, dark planets and dark aliens. At the moment though most physicists look for the simplest explanation

If light is gravitationally bound to an object then you have a black hole so yes it is in a sense invisible. Black holes are a possible form of dark matter, but there isn't enough black holes if you just count how many you would get from stars burning out. You need the black holes to form another way, either in the early universe or out of dark stars (these are called primordial black holes and dark black holes)

 

[grazorblade]

Thoughts on multiverses or cyclic universes given the 'fine tuning' that is present in our universe for it to exist. It sort of defies explanation that the one and only universe that we live in was 'exactly right' the first time a universe existed.

Yes I realise we can only exist because the conditions are just so because here we are BUT if any one of the following were out by even a tiny bit we, the planets and the stars wouldn't have formed.

I grabbed the following from chatGPT because I couldn't remember them all.

Anyone of these out by a smidge and it was curtains. Seems to lend credence that this can't be the only universe that ever was or ever will be.


Yes! Our existence in the universe depends on a delicate balance of fundamental physical laws and constants. Here are a few key factors that had to be "just right" for matter—and ultimately life—to exist:

1. The Strength of the Strong Nuclear Force
This force binds protons and neutrons together in atomic nuclei.

If it were slightly weaker, atomic nuclei wouldn’t hold together, preventing the formation of elements beyond hydrogen.

If it were slightly stronger, nuclear reactions would have burned through hydrogen too quickly, leaving no fuel for long-lived stars.

2. The Strength of the Electromagnetic Force
Determines how electrons orbit atomic nuclei and how atoms form molecules.

If it were stronger, electrons would be held too tightly, preventing complex chemistry.

If weaker, atoms wouldn’t hold together properly.

3. The Ratio of Electrons to Protons
This must be balanced so that the universe is electrically neutral overall.

Even a tiny imbalance could cause repulsive forces to overwhelm gravitational attraction, preventing galaxy formation.

4. The Strength of Gravity
If gravity were slightly stronger, stars would burn out quickly, leaving no time for life to develop.

If it were weaker, stars and galaxies might never have formed at all.

5. The Cosmological Constant (Dark Energy)
Governs the expansion rate of the universe.

If it were too large, the universe would expand too rapidly for galaxies and stars to form.

If too small, the universe might have collapsed back on itself.

6. Matter-Antimatter Asymmetry
At the beginning of the universe, nearly equal amounts of matter and antimatter were created.

If they had been exactly equal, they would have annihilated each other, leaving no matter behind.

A tiny imbalance (about 1 extra matter particle for every billion pairs of matter and antimatter) allowed some matter to survive.

7. The Mass of the Higgs Boson
Determines the masses of fundamental particles.

If it were much different, the universe could have been too unstable or too different in composition to allow complex chemistry.

8. The Right Mix of Elements (Nucleosynthesis in Stars)
Early in the universe, only hydrogen and helium existed.

Heavy elements (carbon, oxygen, iron, etc.) were formed in stars and spread through supernova explosions.

Without this process, planets and life-essential elements wouldn’t exist.

So let's answer each question at a time
1) cyclic universe - this is the simplest attempt to avoid the beginning of the Universe. It also gets theoretical support from the fact that the simplest attempt to make quantum gravity work predicts that there is a repulsive component to gravity at very, very short distances. However, the Universe is not just expanding, it's expansion is accelerating. This is hard to make compatible with a cyclic universe. The other issue is that cyclic universes tend to have each big bang bigger than the previous, so you don't end up with something interesting in the past
2) Yes the Universe is very fine tuned for life, by which I mean that life is possible only within very narrow ranges of parameters. It is a remarkable fact, made more remarkable by the fact that we don't see many instances of fine tuning that' don't have anything to do with making life possible. Yes you are right that one attempt to explain this is to propose a multiverse where every Universe has a different set of laws. When it comes to the challenges of a multiverse it is a different story if string theory turns out to be correct or not.
a) If string theory is correct: there are some big advantages as every free parameter is determined by the value of a field. There are also some real challenges. First, you need there to be a sufficient variety of different Universes with enough different types of laws of physics to explain the fine tuning of our Universe. The exact way you curl up the extra dimensions of string theory sets how much variety you have, and we have to be "lucky" enough to have a large enough variety. I hope this isn't offensive, but I once heard someone tell me the ideal Italian restaurant will have a menu with only a few options whereas an ideal Chinese restaurant has an enormous menu with almost infinite options. You need to curl up the extra dimensions to pull out a Chinese menu of Universes rather than an Italian one. The second, more serious problem, is that string theory struggles to find a way of being compatible with inflation, and finds it even harder to be compatible with the specific type of inflation you need to have in order to build a multiverse. Some colleagues worry that this actually is a severe enough problem to sink string theory, which would be a shame because it really is our best attempt to explain quantum gravity by a considerable margin. One colleague joked that whatever replaces string theory we should just call string theory, because so much time and effort has been invested into string theory
b) if you just have standard quantum field theory you can have the type of inflation you need, so that is good. The problem is that there is no reason in quantum field theory for the laws of physics to be different in each Universe. The only thing that changes without engineering your model is the expansion rate of the Universe. So you have to engineer something unwieldy to get a Chinese menu of Universes. The other issue is that after engineering a potential to make a Chinese menu, you have to worry about our Universe living for long enough. Universes which contract so fast that everything becomes a black hole have lower energy, so the Universe wants to "tunnel" into these Universes. It is quite possible that once you work out all the details you get something as fine tuned as the thing you are trying to explain in the first place. No one knows, because no one has made a concrete multiverse model where all the details are worked out. There is a lot of guessing going on. I was critical of this at first, but found that I also have to simplify things to make progress otherwise it is just to hard and you never publish. Hopefully in a few years people can be more confident one way or another

 

[grazorblade]

Going slightly off track here but this is excellent.

it is an interesting philosophical question whether actual infinities can exist or whether they are incoherent.
Hilbert's hotel is the classic objection to actual infinities and there are more objections these days

since I am not a philosopher I lack the skills to evaluate who is right, so I work with the assumption that the Universe could be infinitely big (though it looks very difficult to make it infinitely old)