Dark Energy, Cosmology part 2: Crash Course Astronomy #43

The previous episode of Crash Course Astronomy
was a bit of a brain-stretcher. We saw that the Universe is expanding, space is expanding,
and it’s carrying galaxies along with it. That means it was denser in the past, and
at some point — 13.82 billion years ago, to be fairly precise — all of space, time,
matter, and energy was compressed into a single infinitely dense point. Astronomers call this
the singularity, which is as good a name as any. Something caused this singularity to suddenly
let loose, expanding violently, cooling, and forming the Universe we see today. Coming
to grips with this idea took a while for astronomers, but nowadays the current working model for
how the Universe started is with a Big Bang. All the galaxies we see are moving away from
each other as space expands between them. That is, on large scales. Remember the ruler
analogy, where on small scales the expansion is small, and on bigger scales the expansion
is faster. That’s why distant galaxies appear to be rushing away from us faster. On small scales, the expansion is small enough
that gravity can overcome it. The Andromeda galaxy, for example, is about 2.5 million
light years away. That means it should be moving away from us as at about 50 or so km/sec.
But because of our mutual gravity, it’s moving TOWARD us; its motion locally through
space is more than enough to overcome the expansion of space between us. It’s like running up the down escalator.
Run fast enough and you can make it to the top. But every galaxy has gravity, and there are
a lot of galaxies in the Universe! That adds up, and should affect the expansion rate.
It’s a lot like the idea of escape velocity: Throw a rock hard enough and, even though
gravity will slow it down, it will escape. But if you don’t throw it fast enough, it’ll
slow, stop, reverse course, and fall back down. Astronomers fully expected to see this effect
on the expansion of space. If you looked on the very largest of scales, you’d expect
to see the Universe slowing down, the gravity of the matter in the Universe itself putting the
brakes on the expansion. And with the discovery of dark matter, that meant the Universe should
be slowing down even more than we first thought! But when they went looking for evidence of
this, what they got instead was probably the single biggest shock in the history of astronomy. In the 1990s, two teams of astronomers were
using the world’s biggest telescopes to peer as deeply as they could into the Universe.
They were looking for incredibly distant supernovae. And not just any kind, but special ones called
Type Ia’s. I talked about these before. They occur when
a white dwarf increases in mass until electron degeneracy pressure can no longer sustain
it against its own gravity. It collapses, undergoes a catastrophic wave of thermonuclear
fusion, and explodes. The beauty of these types of supernovae is
that they all occur when the mass of the white dwarf gets to about 1.4 times the mass of
the Sun; that’s the magic number where pressure overcomes gravity, and they go kablooie. That makes them good standard candles: objects
whose intrinsic brightness, whose luminosity, is known. Knowing that, plus measuring how
bright they appear to be in a telescope, lets you calculate their distances. Then those can be compared to the supernovae
redshifts, which is a different way of getting their distance. This then tells you how fast
the Universe is expanding on really big scales. But the results they got didn’t make sense. Time and again, the supernovae were all fainter
than they expected. It was as if the predictions based on the redshifts were underestimating
the distances to the exploding stars. The astronomers did everything they could to see
if maybe they had made a mistake somewhere, including making a literal list of things
that can make stars look fainter — intergalactic dust, different chemical compositions for
the stars that blew up — all kinds of things. But in the end, both teams independently came
to the same conclusion: The supernovae were farther away than expected. And that meant something truly shocking: the
expansion of the Universe was accelerating. Now remember, that’s nuts. We were expecting
the expansion of space to be slowing down due to the gravity of all the matter in the
Universe. But instead, it was speeding up. It’s hard to overstate how shocking this
is. It’s like tossing a rock in the air, and instead of it slowing down and falling
back down into your hand, it shot upwards faster and faster, defying Earth’s gravity. Of course, scientists were skeptical. Many
still are. But in the end, several other independent measurements have verified this result. The
Universe is not only expanding, but that expansion is getting faster every day. What could possibly cause such a thing? To be flatly honest, we don’t know. Well,
not exactly. But whatever it is acts like an energy suffusing space, pushing on the expansion.
And we can’t see it, so it’s invisible. We already have dark matter, so naturally
this got tagged “dark energy.” It seems to be a property of space itself,
a tiny bit of energy in every single cubic centimeter of the Universe. The amount per
cc is incredibly small, but there are a lot of cubic centimeters in the Universe. It adds
up. And we can add it up. Now that we have measurements
of this, we can take an inventory of the Universe! We can total all the matter and energy in
the Universe, making a sort of budget of stuff in the cosmos. When we do, it looks like this: 95% of the universe is made of stuff we can’t directly see. Normal matter is outnumbered 20 to 1. Maybe
we should rethink calling it “normal.” So if 2/3rds of the cosmic budget is made up of dark
energy, it must have some pretty big effects, right? Yeah. Like, changing the eventual fate of
the entire Universe. A big question – one of the biggest – is,
“Will the Universe expand forever?” Well, astronomers have a framework to answer
this question: We call it the geometry of the Universe. Matter has gravity, and gravity
bends space, so is there enough matter in the Universe to stop the expansion? The geometry
of the Universe mathematically describes its overall curvature, the shape of space on the
largest scales of all. To be clear, this concept is important to
cosmologists, but it can be weird and confusing to someone who is just learning about all
this. Still, a lot of astronomy classes teach it, so I’m going to go over it very briefly, and if you want
more information, we have links in the dooblydoo. The idea behind the geometry of the Universe
is that if there’s enough matter in the Universe the expansion will slow, stop, and
then everything will recollapse; a sort of Big Bang in reverse. If there’s not enough
matter then the Universe will expand forever. And in between the two there’s just enough
matter that the expansion slows, but never quite reaches 0 until an infinite amount of
time in the future. Conceptually, it’s a lot like escape velocity. It was once thought that the geometry of the
Universe, tied to the amount of matter in it, determined its destiny. But dark energy
threw a monkey in the wrench for that, and geometry alone doesn’t determine the eventual
fate of the cosmos. We think there’s enough dark energy in space
to ensure the expansion will continue forever, despite the geometry. Dark energy is just
too powerful, and will always drive the expansion of the universe ever-faster. So, for now, the answer as far as we can see,
is: Yes, the Universe will expand forever. OK, there’s one more brain-melty thing we need to talk
about. For this part you might want to sit down. Space itself is expanding. As light travels
from one galaxy to the next, it fights that expansion, losing energy – just like you
use up energy climbing a staircase. When light loses energy its wavelength gets longer – that’s
what cosmological redshift is. The more distant the galaxy, the faster it
recedes, and the more energy light loses as it travels to us. But wait. At some distance
from us, space would be expanding so quickly that a galaxy in that part of the Universe
would be moving away from us at the speed of light. Anything farther away would be swept
away from us faster than light. Now, before you start complaining, yes, this
IS possible. The speed of light is the ultimate speed limit in the Universe – if you’re
traveling through space. But space itself is exempt from that rule; it can expand at
whatever speed it wants. The matter in it – galaxies, stars, and such – is swept
along with it, so they’re not traveling so much through space as with it. When you solve the equations to calculate
distance and redshift, the distance a galaxy would have to be from us to be moving away at
the speed of light is about 13.8 billion light years. Here’s the fun thing: We can still see galaxies that far
away. We can even see them farther than that. How? It’s because that distance is how far the
galaxy is from us now. When it emitted that light the Universe was much younger, smaller,
and the galaxy closer to us. It would’ve been about 4.5 billion light years away at
the time, and the light took over 9 billion years to get here. Back then, the space between
us and the galaxy wasn’t expanding as rapidly, so the light could keep pace. now, after all
this time, the space between us and that galaxy means we’re moving away from each other
at lightspeed. But back then we were close enough to see each other. For that same reason we can see galaxies that
are moving away from us faster than light, because when they emitted that light there
was less space separating us. The most distant galaxies we see are now about 45 billion light
years from us. We call this the radius of the observable Universe. It’s essentially
the cosmic horizon. Mind you, the actual Universe may be far larger
than this… who knows, maybe even infinite. But we can only see galaxies that are, by now,
45 billion light years away. That’s our horizon. If space were simply expanding, the size of
the observable Universe would expand as well. But we have dark energy, and space expands
more rapidly every day. That means a truly distant galaxy’s light is fighting more
and more expansion all the time – it’s like climbing an ever-steepening staircase.
Compared to a constant expansion, a galaxy in an accelerating Universe has to be closer
for us to see it. It’s losing energy faster. Worse, that fight gets harder every day. A
galaxy just at the cosmic horizon, right on the edge of the observable Universe now might
be visible, but in the future the space between us and it will have expanded even more. The
light can’t beat that expansion, and it’ll never reach us. The galaxy, over time, disappears. This has a weird and unnerving effect: The
observable Universe is getting smaller. The cosmic horizon is approaching us. Eventually
it’ll be so close that every galaxy in the sky except our own will lie beyond it. At
that point our own galactic gravity may overcome the expansion of space, so we’ll remain
intact, but the sky beyond will be black, the edge of the Universe hanging over us just
a few hundred thousand light years away instead of tens of billions. It’s the ultimate irony. The Universe itself
is expanding, but we see less of it every day.
So we’d better study it while we can; we may only have a few trillion more years left
to figure this all out. Today you learned that the majority of the
Universe is made up of dark energy, a currently mysterious entity that pervades space. We
don’t know exactly what it is, but we can understand what it does – it accelerates
the expansion of space. We think this means the Universe will expand forever, even as our
view of it shrinks as space expands faster all the time. Crash Course Astronomy is produced in association
with PBS Digital Studios. Head over to their YouTube channel to catch even more awesome
cool videos. This episode was written by me, Phil Plait. The script was edited by Blake
de Pastino, and our consultant is Dr. Michelle Thaller. It was directed by Nicholas Jenkins,
edited by Nicole Sweeney, the sound designer is Michael Aranda, and the graphics team is
Thought Café.

  • I have a dought. The space is expanding and 1 cm3 has energy if its expanding then its creating more dark energy and we have studied that energy can't be created so where that energy come from?

  • Watching this back to back with the big bang vid of yours, how can the universe be infinately large if it started at one central point, And exploded outward causing the stretch in space and time?

    Im assuming the suns gravity is what is stopping earth stretch apart from the sun itself, its just there isnt enough gravitational pull between galaxys to galaxys, stars to stars to keep them inside the cosmos horizon.

    Again an interesting vid

  • What if the universe isn't expanding, but contracting. Maybe that is why it is speeding up? Like we are getting faster because we are getting closer to the gravitational pull of an unimaginably big, super massive black hole. We know that black holes merge when galaxies eat each other. Which galaxy has eaten the most?

  • So has there ever been any observation of a known galaxy that is suddenly "gone" because it has slipped past the edge of the observable universe?

  • So the universe is expanding. Okay, but expanding into what? What's the area that it's moving into? Does the universe have some kind of border which moves into.. space, stuff, whatever is beyond the border of the universe?

  • Dumb question but is the rate of acceleration constant throughout the universe or does it vary? Like the same way different folds of an air mattress might move away faster from each other than other folds when you inflate it?

  • In an ever accelerating universe, once past the speed of light, a number interesting things start to happen, but are not touched on here.

  • CrashCourse…whata' think about this ? I do.
    The universe is expanding but its expansion is accelerating the timeline also. Since its expanding Relativities light constant is debunked. the universe's expansion does not change the age of the universe as we believe it but if in Hubble Theory the universe is round like a ball and not flat as Einstein sees it not only does the age of the universe change so does its length. (geographical length) A hundred years ago sailors measured the length between landmarks using a fold-page which showed the length of measurements curving and not seable or whats around the corner and bringing it to view on a flat universe flat world. ( For instance, light (quanta) lurking around the corner changes into e/m waves) Albert Einstein used the mass of the length of light '' meaning the mass affected by gravity 93,000 m/s changed the length with billions of light years and called that the length of the flat universe. Erwin Hubble than proved Einstein was wrong when he discovered the universe was expanding at a speed greater than light. Though no scientist has recalculated that length over history from 19010 Hubble showed the universe was round like the Earth was not flat. Relativities debunk is just that – a debunk and it affects all his work even the age of the universe – but dont take my word for it) Like your video you get a ten….
    How can something so wrong be so right – in physics? So the legend plays on ! That's an origional,,,?

  • If all the matter that falls into black holes was converted to energy, (E=MC^2) would that total be close to the 63% of unexplained dark energy?

  • Can anyone explain how can I understand all these concepts including quantum mechanics & relativity with super ease but hate basic mathematics?!

  • A civilization that rise in about 10 trillion years from now will think that their home galaxy is the only one in the universe.

  • So 'dark energy' has never been observed and simply seems to be more of a concept that seems to explain things we can't otherwise explain. Yet, that 'dark energy' is then used to explain other things. Seems like we're building castles in the air.

  • The expansion isn't speeding up, but rather is infinitely big.
    Infinity is the concept of there always being a higher number/other value. It's the same with the universe. It doesn't expand, since that would mean there was something to expand into. The further you look into the cosmos, the more things you see.
    Another theory I have also says, that since matter can never be made or destroyed, all matter is stretching, if in fact the universe is not infinite. If all matter gets "stretched", the gravitational pull on our universe gets smaller and smaller, just like stars and black holes, etc. Not sure if the last one was just me not listening carefully enough, but I like the theories, nonetheless.

  • Until the expansion gets even closer to the solar system, planet and eventually subatomic particles ? Each particular forever isolated.

  • Ironically mankind believed we were the center of the universe. New civilizations trillions of years from now will come to the same conclusion and have no way of telling they are not correct

  • So if the universe is constantly accelerating but matter can never reach the speed of light then it infinitely accelerates while also not allowing it to reach or exceed a finite speed…wow

  • قال تعالى: ﴿وَالسَّمَاء بَنَيْنَاهَا بِأَيْدٍ وَإِنَّا لَمُوسِعُونَ﴾ [الذاريات: 47]

  • Thanks so much for this video. Answered what I have wondered for years. If D Energy is pushing everything away faster why are we going to collide with Andromeda. Duh… gravity. Should have known. So it leads me to another question. Does this mean we will devour the dwarf galaxies around us first? They are closer to milky way than Andromeda is.

  • He said the most distant galxies we see are 45 billion light years away. Universe is 13.8 billion years old. Well someone explain .

  • Everyone sad about us possibly losing the ability to see the rest of the universe.
    Phil says we only have a few trillion years left.

    Me: I’ll be dead before that happens. Sucks for those losers alive at that point, I guess.

  • I wish you hosted all of crash course. the other host who presents world history just gets lost in appealing to kids and using comedy to break up information way way too much!

  • If we could leave our galaxy's gravitational influence and enter space that is stretched by dark energy, would it have a physical effect on us?

  • If the universe is expanding then wouldn’t we move with it as well? It’s like 2 cars on the highway they’re both moving certain directions but they still can see each other and other cars in the opposite lane are getting farther. So really the visible universe is changing not getting smaller

  • Well, here's a crazy idea from someone who knows very little about astronomy: what if everything in our universe is simply being sucked into different universes that are going through a big crunch? So these other universe's gravity is what's making our expansion accelerate.

  • If the universe could expand and also contract. Wouldn't there be a point where it DOES stop expanding and we begin to see matter traveling back toward us?

  • Have you (Phil Plaitt or any other pro astronomers out there) ever seen one of the galaxies at the very edge of the horizon suddenly pass beyond visibility and disappear? Man, that would be fascinating to see, and to know it's still there, we just can't see it any longer and never will again. Astronomy is truly fascinating- you get to see the actual past with your own two eyes in what is right now to us. What a thing to try to wrap your head around huh?

  • If space is still expanding and accelerating, then I would theorize that the Big Bang hasn't completely fizzled. When something explodes, the debris ejected outwards is being pushed by enough energy to overcome gravity. Once that energy is expended, the debris slows down and falls back down. The explosion just isn't over yet.

  • One question I have is if our local group will survive the expansion of the universe. Since we are already close enough for gravity to overcome the power of dark energy expansion, is that an absolute for all of eternity, or will even our own local group be lost to the cosmos however much time in the future?

  • Thanks! You just proved to be the source I needed to finally know what to write about for my physics paper. 🙂

  • If we don't know what "Dark Matter (gravity)" is and we cant see it but we know it's there because of the effects of it – do we know if this dark gravity has a finite amount or is it "born" and if so wouldn't the growth of more dark gravity make everything around it be pushed aside i.e. expand the universe? If it keeps growing and getting pushed aside and that pushed-aside matter multiples and this repeats wouldn't that explain the increasing rate of expansion?

  • Not trying to sound controversial, but an observation. In the video (or previous part), It states that there was nothing before the universe, and all of a sudden, the cosmos came into being within factions of a second (pretty much instantaneously). Essentially creating something from nothing. Moving onto Genesis 1:3, it states, "Let There Be Light". Before then, there was only darkness, and as soon as it was said, Light poured out from nothing. With that logic, the Big Bang doesn't discredit the Biblical creation narrative, but in fact might actually enforce it.

    Edit: If you have counters to my argument or know something I forgot, please let me know. I really want to understand the whole idea fully. Please though, don't act like trolls, I just want to have a civil conversation/discussion.
    Also, I'm looking only that the beginning of the universe, not on the Biblical portion of the creation of earth or any Christian ethics. Just the Beginning of the Universe.

  • This is so depressing. Our hair falls out, our bodies die and our earth will die one day, the universe is going to be lonely this is so so depressing.

  • the earth is flat. the sun revolves around earth and is the size of the moon. and you my friend are wrong

  • ok, he said that the universe is 14 billion years old, then can someone explain how we can see a galaxy 40 billion light years away?

  • Can’t this explain away the Fermi Pardox if it’s getting increasingly more difficult to see/communicate with far away galaxies?

  • The Quran suggested this 'expansion' and 'disappearing of stars' 1400 years ago! I'm happy to have run into this scientific explanation – my heart is even more certain now on the validity of my beliefs 🙂 Thanks for the video…

  • NASA claimed that Hubble Space Telescope can see the young universe which implies that celestial bodies were travelling much faster than the speed of light in the past but somehow they have slowed down for some time slow enough now for the light of the past to catch up with us that we manage to see the young universe. The expansion of the universe is slowing down to pave way for imminent implosion to facilitate for a next round of Big Bang. Halton Arp discovered that different portions of a galaxy can have different red-shift readings with dimmer stars have higher red-shift readings than the one of brighter stars. Generally, stars at the edge of observable universe are very dim therefore they most likely to have very high red-shift readings and this doesn't imply that they are retreating away from us at accelerating rates. Relativity theory is wrong since things can actually travel much faster than the speed of light. Speed of light is not the limit. Sky's the limits. Where on earth that it is sitting on the gravitational wave that is inaccessible to anyone? None. Don't tell me that flimsy atmosphere can support the weight of the earth in causing it to float above  the gravitational waves! Therefore the existence of gravitational waves is a bluff. If you are interested in real discoveries, I would recommend you to read my book, The Unification Theory – Volume One and you will be amazed with lots of new, interesting discoveries.  In God I trust.

  • So if I get this right, he's saying that the universe is growing so fast that as the all the galaxies spread out farther away from us, we'll eventually not be able to see them outside of our own galaxy since the light won't be able to reach us?

  • A question though: When we look deep into the space, we also look into the past – the light/wave we collect from other cosmic objects were from light-years away. So when we say the universe IS expanding – shouldn't it more accurate to say "the universe WAS expanding, and we don't know if it is STILL expanding"?

  • One thing on my mind… OK so I read that the entire universe, at a trillionth of a second, used to fit within Earth orbit.  So it was way, way, way denser than any black hole ever seen.  So nothing "should" be able to escape it, right?  Yet the entire universe did.  How?  If I find any answers, they are weirder than anything in any Book of Revelations.  "Well, back then gravity did not exist, it was not switched on, mumble mumble".   Does it make sense to anyone here?

  • If the dark energy is cause the expansion, that means it's energy which is getting converted in kinetic energy of galaxies which is mass, which Means the acceleration is basically decreasing and after certain amount of time they won't be accelerating rather will be moving with a constant speed
    Which sure should be the greater then the speed of light as we know how powerful is dark energy and mass is really small as compare to it.
    Any thoughts on this ?

  • Are there any examples of astronomers observing a dim object at the edge of the cosmic horizon one year and then the next year not seeing it anymore? Now THAT would blow my tiny ape brain.

  • "That would be a cosmic joke," the skeptic told the bartender.
    "What?" the bartender inquired.
    "If dark energy is nothing more than mind."
    "How's that?"
    "You can't see or measure it, but there's no doubt it moves matter around."

    The bartender smiled as she responded, "It's also what fills many of my customers' heads."
    "What's that?" the skeptic asked.
    "A lot of empty space."

  • How do we know that the universe is being pushed apart by an internal force and not pulled by an external force. Or, even, shaped in such a way that gravity is pulling us apart rather than pulling us together.

  • The Big Bang lambdaCDM Model is so full of “boot strapped” fixings that render it unacceptable and in fact …horrible and ugly!

    Think about this …96% of the theory is missing! Dark Matter and Dark Energy!! Lol 🤷🏻‍♂️

    Why is scientific community so adamant with the theory? Because the only alternative is the Geocentric model that works beautifully…and without those made up Dark Energy and Dark Matter fairy tales! 👍🏻

    Genesis 1:1😊👍🏻

  • A couple of vids ago the discussion stars collapsing, more specifically neutron stars. I may have misunderstood this but I thought he said that when the star starts to collapse, assuming more than 1 1/2 masses of our sun, the collapse is so fast that it was faster than the speed of light. Now, whether I mis-remembered that or not, I questioned how the speed of light is the universal speed limit but a star could collapse faster. Now he just said that the speed of light is the speed limit but space can expand faster, thus making things move faster than the speed of light. Maybe I did get the star collapse thing wrong but if not, now I understand it a little better. Thanks!

  • Dear DR plaite,
    Would it make more sense to think of a number of multidimensional soap bubbles intersecting and forming our universe with its set of rules. And like colliding galaxies our universe is a open system that is dynamic and we are currently experiencing expansion because the soap bubbles that cause our intersection (universe) is creating the strangeness. And maybe dark energy and dark matter are artifacts caused by high dimensional objects that we can only experience through its gravitational affect. What if thats what causes us to misqueue a jump or step and causes accidents. Phase shifted gravitational sources where light does not interact, what if all matter has more dimensions too it than we think, what would we look like through a 4-space or 5-space perspective, how twisted would perspective look outside of our 2D+ perspective (because EM plane waves). Even imaging 3-dimensional coupling between objects within the Near-field EM is difficult, imaging gravity is stranger, imaging nuclear force flux also difficult.

  • I like that the universe has made my brain to get really unconfortable with space being infinite…. and space not giving a single hoot about what my brain thinks.

  • Can anyone explain this:
    There is an assumption that dates back 1500 years ago where in the Muslims' holly book, the "Quran", Ch : 51 verse 47, God says: "And the sky (space) We constructed with strength, and indeed, We are [its] expander"

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