GMO that's cool.
Astronomy & Space Exploration
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I know. However, there are subtle differences between them. It is widely believed that dark matter is matter, just that it does not interact with the ordinary matter. It would be a bit like neutrinos in steroids (just for the analogy). Neutrinos, for the most part, don't interact with the ordinary matter, but at times it is possible to detect them. Essentially, the probability of them interacting is low, but not zero. I think that scientists believe that something similar might be with the dark matter, just that the probability of interaction is much smaller. And they are confident that the dark matter is there, otherwise the gravity would have been behaving differently. Essentially for the gravity to work as it works, there need to be a lot more matter right there. But there isn't. And here, comes the dark matter. Actually, the matter is right there, but we cannot see it, we cannot detect it, but we can see its effect on gravity.
Dark energy is very different. We cannot see it, we cannot detect, we have no idea what it might be (matter, energy, something else*). But we know that considering only the matter and the dark matter in the universe, the inflation wouldn't be working as it works now. Everything is going further from each other with increasing speed. There needs to be something that is making the universe behave in this way. And so, the physicists invented the term of 'dark energy', essentially, the phenomenon that is making the universe behave this way.
I might be wrong, but I think that the common belief is that dark matter might be closer (more similar) to the ordinary matter than to dark energy. We kinda know what dark matter might be. We don't know at all what dark energy might be.
* Again, I might be wrong (I am not a physicist), but the amount of dark energy in the universe seems to increase all the time, to the point wherein future, close to 100% of energy/matter in the universe, would be dark energy. So, it is very likely that dark energy is something much more mysterious than just energy we cannot detect.
Yes that's my understanding too, that dark matter is much more defined than dark energy, as some sort of matter which only seems to interact gravitationally. Dark matter almost definitely exists, while we mostly just need Dark energy to explain some theories that might still turn out to be wrong (but probably aren't).
We're pretty sure galaxies exist within Dark matter halos, for example, and it might help explain how galaxies formed in the first place. There's also graviational lensing, at least some of which can't be explained with just ordinary matter.
It's all just very interesting, even for a lay person.
We're pretty sure galaxies exist within Dark matter halos, for example, and it might help explain how galaxies formed in the first place. There's also graviational lensing, at least some of which can't be explained with just ordinary matter.
It's all just very interesting, even for a lay person.
I am a big believer in the multi-verse theory. I always say our universe.
The concept that the universe started from nothing is no longer mainstream thinking.
That is mainly because of the law of energy conservation.
So all the energy in our universe had to come from somewhere.
I have read about and tend to believe in the theory that baby universes are infinitely being born. The so called bubble universes.
Some fail to form and release their energy back into the bubbles.
Others, including our universe become viable and then develop in an infinite number of forms.
Ultimately, they die and release their energy back.
My primary view is that there are so many things during the early stages of our universe that are required to happen to make it possible for galaxies and everything else to develop that it was either luck, or inevitable.
For a lay person, you have an excellent grasp of the science. As do many others who post here.
Definitely the most interesting thread.
Thank you, it's a very stimulating thread, and topic in general. I love reading about this stuff, and watching the odd lecture/talk on YouTube. The math goes right over my head usually, but I try to cope!
Indeed.
I have to admit that I too struggle with the maths. It has never been a strong point.
I did get a physics A level though which was a big surprise, albeit many decades ago.
And that is why I love the books by Brian Cox so much. He very much concentrates on the science and where there is a need for maths, he breaks it down to that which is absolutely necessary.
Is the paper behind this available somewhere ? All the articles I read on this seem a bit sketchy on the details. What particles specifically are the 'heavy particles ' ? It is also kind of implied (maybe I misread) that the 'heavy' particles were directly detected by the instrument but If I understood correctly it detects photons in the radio spectrum emitted by the interactions of the space particles with the electrons on the surface.
When physicists talk about "nothing", its not our nornal concept of nothing. It means empty space in thermal equilibrium. It could be a previous universe that is dead. Then some quantum fluctuation in its empty space started our big bang. But the laws of Physics would need to already exist, so is not really nothing, its a lot of stuff.
An universe coming from that kind of nothingness would not violate conservation laws anyway. First because the positive energy in the matter balances the negative energy in the gravity. So our universe could come from nothing because it has zero energy. Also conservations laws are localized approximations, not necessarily apply to the universe as a whole. You can violate conservation laws if you do it very quickly. Particles do it all the time emitting virtual photons to interact with each other.
There are many versions of multiverse, some emerge from inflation, other from trying to make sense of string theory. There is also the so called many worlds interpretation of quantum physics. I think multiverse is more mathematics than science. But the arguments for its existence are very solid. I tend to believe.
The many worlds interpretation is probably not the type of multiverse he refereed to and the name itself can be a bit misleading. Whats the solid (mathematical) argument for the existence of a other multiverses?
Very interesting. Thank you.
The point I was trying to make in the universe out of nothing theory was that, we are told that before the big bang, nothing existed.
No space and as a result no time.
I appreciate that quantum physics does not permit there to be absolutely nothing at all.
I go along with the cosmic inflation theory. Primarily because it seems to answer a number of the anomalies of our early universe (not that I completely understand it but far bigger brains seem to).
And it is now standing the test of time.
Many worlds is obviously a multiverse. The term "world" is that is misleading. It should be many universes. Unless we count the whole wave function of all possible "worlds" as only one big thing. But in this case we need to apply the same logic to any version of multiverse.
In many models of inflation, once started, it will go on forever creating an infinite number of universes. The mathematics of this model predicted the size and shape of the fluctuations that left a fingerprint in the CMB. I do not agree with with string "theory", but its multidimensional landscape also suggests the existence of a multiverse to explain why our universe have this specific laws.
I go with inflation too. There are some push back against the theory recently, but i have an emotional link, because it was the first science text i read when i was young
Yep basically dark energy is proportional to the amount of space there is so as it expands the more dark energy there is. 7 billion years ago was the time that dark energy became dominant and it's the reason why physicists believe in the heat death of the universe theory.
We know more about dark matter for sure we can see its effects in the rotation of galaxies and gravitational lensing etc. Also they run super computer simulations of the birth and evolution of the universe with and without the calculations for dark matter and they only resemble our universe with dark matter.
Really interesting.
Especially the bit about Dark Energy increasing.
Assuming the laws of energy conservation apply to dark energy, it must be taking or absorbing energy from another source.
Again, assuming that to be true, you would have thought that we would be able to detect that difference.
Not on a global scale of course, but in an experiment.
Anyway, quite fascinating stuff.
conservation of energy is tricky in general relativity, because space isn't an "independent background" for particles in this theory. Conservation laws (e.g. energy, momentum, angular momentum, charge, Baryon number) are connected to symmetries. When one interprets dark energy as cosmological constant (~energy density of empty space), than space-time isn't flat. Without flat space-time, neither momentum nor energy are conserved globally. There are still understood rules how energy evolves in GR. The energy of an expanding universe can increase, just like you intuitively and correctly assumed (Another example of this is photons losing energy in expanding space due to red-shift).
Now you might read articles/accounts that state, that energy is conserved after all, which seem to contradict what I just wrote. It all depends on the definition of energy. There is a definition, where energy is conserved, but that definition includes additional stuff.
There is no disagreement about what happens, just how to explain it. Imo the easier and more truthful understanding for the layman is, that conservation of energy doesn't apply to our expanding universe.
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Even more interesting and thank you for taking your time to explain it.
I will have to read this a few times before it starts to make sense to my old brain.
Just for clarification.
When you say that the energy of an expanding universe can increase.
If increasing energy is making the universe expand, and the expanding universe can increase the energy, what is the source of all that energy.
And where does it come from.
Reason for asking is that I had understood that all of the energy in our universe was there at the beginning.
Or have I misunderstood the science.
Thank you.
If you consider the dark energy as energy, then no, not all universe’s energy was there at the beginning. In fact, with universe expanding, its total energy (because of the dark energy) is ever increasing (and in every second, it increases more than in the previous second).
Why and how this happens? I have no idea (and I don’t think there are many good hypothesis to it). Probably the best I have read somewhere (or I have imagined it) is that the dark energy is the cost function of space time. So with space time expanding, so does the dark energy.
The scary thing (if you can think as scary about something that is gonna happen trillion of years from now) is that with the process going this way, at some stage every particle (quark) would be extremely far away from any other particle. The entire universe would be a dark empty space and on limit, every mass/energy would be dark energy.
We're talking way more than trillions of years here, aren't we, if you're referring to the heat death of the Universe?
The big heat death of the Universe is estimated to happen within the next 100 trillion years if I am not mistaken.
What I was talking though was about the Big Rip (aka, die by the ever-increasing expansion). The estimates vary there, but they can be as early as within the next 20 something billion years, and probably up to a few trillion years. So if this happens, then there won't be at all a Big Freeze (heat death) cause the stars, planets and ultimately atoms would be already destroyed from the acceleration and we won't ever reach the stage of new stars not having enough material to be formed.
NB: Again, this theory assumes the existence of dark energy, in fact, of a very particular version of it (which among other things, has negative mass). All dark energy theories might be totally wrong, they exist only to make the theory of general relativity (and by extension gravity) work with the actual data. A new theory of general relativity might be discovered without the need of it to have dark energy.
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Oh right, Big Rip. I remember when Big Rip was the big thing (no pun intended), is it coming back now?
Regarding the heat death of the Universe, as far as I remember 100 trillion years refers to star formation, "stuff" is going to stick around for a whole lot longer than that. At that point there's for sure not going to be any new life, but I always liked the idea of some hyper-advanced civilization managing to continue on around a black hole.
Regarding the heat death of the Universe, as far as I remember 100 trillion years refers to star formation, "stuff" is going to stick around for a whole lot longer than that. At that point there's for sure not going to be any new life, but I always liked the idea of some hyper-advanced civilization managing to continue on around a black hole.
Yep, covered all the bases and that is my understanding as well. Couple of key conjecture(s):
1. Dark energy is an intrinsic consequence of space-time itself and the energy is uniformly dense over a given portion of spacetime in the grand scheme of things.
2. New spacetime has and is being dynamically created since the inflationary epoch ultra-expansion (when sub-atomic scale distance became galactic-level distances in like 10^-30s).
3. Hubble/Lemaître observation, the further things are...the more red shifted they are, indication of increased distant inflation.
Briefly...
• In the very-very beginning, spacetime was infinitesimally small, and the effects of dark energy were diminished by matter and relativistic soup.
• Immediately after that, matter was in ascendancy — gravity coalesced matter into more matter-dense regions and retarded the creation/inflation of space-time (especially within pockets).
• As space-time continued to grow (slowly compared to now because of matter), matter and dark energy reached a tipping point roughly 4-6 billion years ago — since then spacetime (and consequently dark energy) are being created in an uncontrolled chain-reaction of sorts.
Unless there's a fundamental phase transition, this universe is monumentally fecked under the eternal inflation model — as for humans, the tour was over before it even began as it is impossible for us to ever, ever, ever travel beyond the cosmological event horizon (we won't make it that far but even if we did at this very moment for the sake of argumentation, it would be futile as you would need to go faster-than-light to traverse beyond the boundary).
You're right, up to 100 trillions is only for the stars. For the black holes to decay, we are taking for a significantly longer amount of times, for the supermassive black holes, apparently something like 10^100 years (https://medium.com/starts-with-a-bang/how-do-black-holes-evaporate-5463dbda6832). That is quite long to be fair.
I thought that Big Rip is the big thing. Providing that the dark energy theory is right, Big Rip is gonna happen. Of course, the theory might be totally wrong. On the other hand, the heat death of the universe is likely true, just that something else might happen before.
I am sad, even if we crack immortality, it is gonna be difficult to stay alive after 10^100 years.
Yep, more or less this. In really simplistic terms (a theoretical physicist would kill me), if the dark energy is a property of space (so every m^3 has x amount of dark energy, I think estimated at 10^(-27) kg) and this is uniform, then there is a chicken and egg scenario. The more universe expands, the more dark energy is 'created'. The more dark energy is there, the faster the acceleration of the expansion becomes. Until at some stage, both the acceleration and the space become infinite, and the universe is essentially dead (any particle is infinitely far away from any other particle, essentially making the universe an infinite ball of nothingness).
Just going by Wikipedia (always a safe bet when it comes to astrophysics
), and from what I remember from other sources, the existence of dark energy doesn't necessitate Big Rip. Rather, it's one of the possibilities, and depends on the properties of that dark energy.I'm hoping for Big Freeze instead, I'd like to be alive for a few quintillion years thanks.
True.Just going by Wikipedia (always a safe bet when it comes to astrophysics
I'm hoping for Big Freeze instead, I'd like to be alive for a few quintillion years thanks.
One of the variables in the equation needs to have a value smaller than -1 for it to happen. If it is -1, it takes an infinite amount of time for it to happen, if it is smaller than -1, then it takes a finite amount for Big Rip to happen (and the exact moment when it happens, it depends on it, so the smaller it is, the faster the universe dies).
Yeah, me too. No way I am giving up living in a few billions/trillions of years, just cause the physicists decided to add some equations (which cannot be proven to be wrong, and well, we haven't proven them to be right) to make general relativity work.
On other news, thanks @Invictus for giving me the 10th like, which in turn gave me the 'Promoted' trophy. I guess I finally got promoted 8 years after being promoted :P
BTW for anyone interested, this channel is pretty informative, and this video in particular does a good job of demystifying the Big Rip hypothesis for your garden-variety enthusiast:
Its mostly like you think it is. Kind of. Energy or matter don't just pop up out of nowhere.
Lets forget dark-energy for a second. Its important to think about the framework which is used to look at a problem. Conservation of energy is true in Newtonian mechanics, special relativity and quantum mechanics. Its problematic in general relativity (GR).
In GR spacetime and matter interact. The curvature of spacetime depends on the matter&energy and that breaks time-translation invariance. Time-translation invariance means roughly *the background in which particles and forces propagate is fixed* and that is a condition for the law of conservation of energy. In GR, thats not the case. Spacetime can evolve and change. Its understood how spacetime can influence energy; it doesn't happen randomly or is super mysterious. Its absolutely crucial to understand many things in the first minutes of the universe. Without taking this into account, we'd get it wrong.
Another problem/perspective of the problem is, that "book-keeping" in GR is problematic. In Newtonian Mechanics one could imagine to take the position of a "outside-the-system" (quasi-god-like) observer to keep track of all the energy. There is a clear answer to the question "how much energy is in our visible universe", that is true for everyone in this universe. In GR this doesn't apply. Your relative position matters. Observers, who move differently in regard to each other would get different answers. There are many more issues and all of them can be addressed in one way or another, but the resulting conservation law includes a lot of caveats to the point that its probably more confusing.
Now lets get back to to dark energy:
I have written the following two posts in 2017 about dark energy/matter in a slightly different context:
I think the best way to understand dark energy is to say that its a property of space itself. There is always the temptation to think about the expansion of the universe/space itself in terms of an explosion, but thats the false picture. The balloon-analogy is more useful. Once its there, it just has a certain vacuum energy associated with it. It makes intuitively sense, why this would add energy to the universe even if we don't really understand the detail yet. It remains to be seen if that view of dark-energy is true and there are other theories, some where dark-energy is not a constant property of space but more field-like and (potentially) changing (e.g. dark energy is a Scalar-field (like the Higgs-field)). These are imo even more speculative.
In regards to conservation of energy its imo best to think, that this is not a strict principle in cosmology and there are certain very special processes related to the expansion of space (like dark energy or redshift), where the energy can change. If its unsatisfactory to think that conservation of energy is a bit less reliable in GR, you can adopt the more technical point of view, that these changes are getting canceled out by a change of the gravitational field (which can/does have negative values). I think its deceptively hard to understand what that actually means and doesn't add anything in terms of understanding what happens.
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Really good posts, @PedroMendez
One thing though that I struggle to understand, is why the speed of acceleration increases if the dark energy amount per unit space (whatever you define it) is constant. Sure, the total amount of the dark energy in the universe increases with the expansion of the universe, but how does this energy in turn make the universe expand faster? Surely, it somehow needs to be related to the expansion of the spacetime (rather than the spacetime itself).
One thing though that I struggle to understand, is why the speed of acceleration increases if the dark energy amount per unit space (whatever you define it) is constant. Sure, the total amount of the dark energy in the universe increases with the expansion of the universe, but how does this energy in turn make the universe expand faster? Surely, it somehow needs to be related to the expansion of the spacetime (rather than the spacetime itself).
No your right the amount of energy is the same and has to be the same due to the 1st law of thermodynamics now dark 'energy' increasing seems to violate that but we know so little about it that we cant class it as matter/energy in the traditional sense.
Otherwise as the universe expands and as such more dark energy is in place then normal matter/energy would have to disappear which it doesnt.
We only call it dark 'energy' because it seems to have an exponential effect on the expansion rate of the universe outside of that we really have no clue about it. Is my understanding anyway.
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Guys.
So much fantastic information above. I am in awe of all of you for your incredible knowledge of this highly complex and ever evolving subject.
What an amazing thread this is. My brain is really struggling to make sense of it. But I will re-read each of the posts and try to let them sink in.
Each time I think I have got a basic grasp, something else makes me question my understanding.
But at least I now realise that, at least in theory, dark energy and space time are intrinsically linked.
So much fantastic information above. I am in awe of all of you for your incredible knowledge of this highly complex and ever evolving subject.
What an amazing thread this is. My brain is really struggling to make sense of it. But I will re-read each of the posts and try to let them sink in.
Each time I think I have got a basic grasp, something else makes me question my understanding.
But at least I now realise that, at least in theory, dark energy and space time are intrinsically linked.
The expansion (or contraction) of spacetime depends on the relative proportion of the energy density of normal + dark matter, radiation and dark energy (+curvature, which I'll just ignore here).
The total energy of normal+dark matter is constant, but the their energy density is falling with increasing volume of space. The energy density of radiation is also falling with the volume of space and is additionally redshifted (=>decreases even faster). The energy density of dark energy is constant (if its interpreted as vacuum energy; in other interpretations its not fixed, but decreases way slower than the other parameters). With ever expanding space, the influence of of matter+radiation goes against zero, while the expansion is depends only on the energy density of dark energy.
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In the end its primarily based on observations. Einstein introduced the cosmological constant as a parameter to "fix" his his field equations. He thought that the universe is static and assumed he needed this extra term to make it work. He was mistaken in more than one way and shortly afterwards Hubble found evidence, that the universe is not static. My point is, that this part of the equation was less motivated by theoretical understanding and more by "how to make it work".
The same remains somewhat true for dark energy. The measured expansion rate can't be explained without adding something. One can quantify what has to be added to make it work in the context of the established framework, but there isn't much solid theory behind that. It seems to be that way, but who knows why.
I tried to explain to mum in the pub before I was told enough dan.
I said imagine a sheet of grid paper with a circle on it and to imagine the circle growing bigger and bigger, to explain dark energy increasing as the universe expands.
(The squares on the grid being dark energy, and the circle being the ever expanding universe.)
I said imagine a sheet of grid paper with a circle on it and to imagine the circle growing bigger and bigger, to explain dark energy increasing as the universe expands.
(The squares on the grid being dark energy, and the circle being the ever expanding universe.)
Can anyone link me to a good articles about this new universe where time is going backwards please
https://www.independent.co.uk/life-...reverse-nasa-physics-antarctica-a9526226.html
any good article should tell you that its random speculation, which is vastly overstating highly speculative and completely unsupported ideas to generate clickbaity titles.
Like the rest, I also think these last few pages have been great.
I've been listening an astronomy podcast for a while and curiously these last months they are especially interested in matter and dark energy.
In any case, I have a question.
I have heard in this podcast, on YouTube and here on Red Café: @Invictus , in another thread "or all we know, this universe could be a lab experiment in a wider cosmos which is why we have such elegant physical laws".
what exactly means the term elegant?. To the precision of the laws? How do they fit together? or rather refers to how they are fair to promote life?
I've been listening an astronomy podcast for a while and curiously these last months they are especially interested in matter and dark energy.
In any case, I have a question.
I have heard in this podcast, on YouTube and here on Red Café: @Invictus , in another thread "or all we know, this universe could be a lab experiment in a wider cosmos which is why we have such elegant physical laws".
what exactly means the term elegant?. To the precision of the laws? How do they fit together? or rather refers to how they are fair to promote life?
I will leave it to those far more knowledgeable than me.
But as I have read a lot about the events during the very very first fractions of a second after what we term the big bang, it has always fascinated me how different our universe could have become.
The list is long. But would include:
Cosmic Inflation
Matter v Antimatter
How the Unified Force split.
The relative strength of the four forces.
I am sure there are many more.