Why the Universe Needs Dark Energy | Space Time | PBS Digital Studios

Why the Universe Needs Dark Energy | Space Time | PBS Digital Studios

Can we try to predict
the fate of the universe? Amazingly, yes, we can. Yet our efforts to
do so have revealed the existence of a strange
influence called “dark energy.” [THEME MUSIC] Einstein’s general
theory of relativity allows us to
describe the behavior of the cosmos on the largest
scales– of a volume of space vastly beyond our
capacity to physically explore and at a time billions
of years in the future. But attempts to constrain the
predictions of this theory with real astronomical
observations tell us that the
fate of the universe is governed by something
we call “dark energy.” To truly understand
dark energy, we’re glimpsing into the workings
of Einstein’s theory on a cosmic scale. In the last episode,
we looked at how general relativity, expressed in
the first of the two Friedmann equations, can describe
the cosmic balance between the outward expansion of
the universe and the resistance to this expansion due to
the gravitational effect of everything it contains. Seriously, you have to pause
now and watch this episode if you haven’t already. OK, ready? So it turns out that
this cosmic conflict will be won by expansion. And this is true even though
we haven’t taken dark energy into account yet. By weighing up all of the
matter in the universe, astronomers have figured
out that there just isn’t enough of anything to turn
the universe back on itself. There will be no Big Crunch. And the universe
will expand forever. There are two powerful
and completely independent measurements that can
test this prediction of the fate of the universe. Today, let’s look at one
of these measurements– the geometry of
the universe, which points to a discrepancy in
the first Friedmann equation. We know for sure
that, the left side of this equation, the sum of
the expansion and density terms don’t cancel out to zero. In fact, they come
out positive, which means that the
expansion term wins over the inward-pulling
density term. The universe is too
low-density to recollapse. Right. So the left side of this
equation, as we wrote it, is positive, which means the
right side should be, too. See? Equals sign. But that right side actually
talks about something completely different. It describes the
curvature of space. And it hinges on that k thing. k is, in a sense, the
shape of the universe– its spatial curvature, as well
as its spatial extent, finite or infinite. Notice I said
“spatial” curvature. Spacetime will be
curved, no matter what. Which is what gives us the
expansion described here. But the spatial geometry of
the universe at a fixed instant in time can be flat or curved. And it’s pretty straightforward. k can be plus 1,
minus 1, or zero. “k equals plus 1”
means the universe has a positive-curvature
spatial geometry. Spatial snapshots of the
cosmos at an instant in time will be curved like the surface
of a sphere– except the 3D surface of a 4D hypersphere. In a universe like
that, geometry is weird. The angles of triangles add
up to more than 180 degrees. And circles and spheres contain
more internal area/volume than their circumferences
or surfaces should allow. But the total spatial volume
of such a universe is finite. And we call that a
“closed geometry.” If k is minus 1, the
universe is the 3D version of a negatively curved
hyperbolic plane. Triangles add up to
less than 180 degrees. And surfaces hold less
volume than in flat space. Such a universe is
infinitely large, or “open.” But “k equals 0” means
the universe is flat. Zero spatial curvature,
at any given time. Geometry works just as
you learned in school, and a flat universe is still
infinite– open– in all three spatial dimensions. But check it out– this gives
us a way to independently verify what we got on the left side of
the first Friedmann equation, when we measured the
universe’s density. The left side has
to equal the right. So, assuming we got
the equation correct, then the shape of the universe
should be intrinsically tied to its fate. An overdense,
recollapsing universe should have a
spherical geometry. An underdense, infinitely
expanding universe should be hyperbolic. And only a universe with
exactly the right density, that’s expanding at exactly
its escape velocity, one that will slow to a stop
over infinite time, should be flat, with the both
left and right sides coming out to zero. OK, cool. That’s something we can test. See, we can measure the
shape of the universe and, so, measure k, just
by checking how geometry works on cosmic scales. Now, if we got
everything right so far, the geometry should turn
out to be hyperbolic. Because that gives us a
positive right-hand side to match the positive
left-hand side we got from weighing
our universe. In fact, we already talked
about this in our episode on inflation. Check it out. Observations of the size of the
cosmic [INAUDIBLE] background features allow us to
verify that the longest triangles in the
universe have angles that add up to exactly 180 degrees. That’s the straight-up geometry
of a flat, Euclidean universe– flat to within 0.4%. The right side of the
first Friedmann equation has to be very close to zero. This is totally
inconsistent with the level of positive curvature we’d
expect from an infinitely expanding universe. So, did we miss up
our measurements? Is general relativity wrong? No. But when we tried to describe
the universe by reducing the Einstein field equations
into the Friedmann equations, we missed something. We missed the
cosmological constants. We also talk about
this in inflation. The very same addition to
the Einstein field equations that can describe
cosmic inflation can also fix this little
problem with the first Friedmann equation and geometry. When we derive the
first Friedmann equation with the
cosmological constant included– that’s
the lambda symbol, here– we end up with this
little extra something on the left. Assuming the
cosmological constant is positive, this works on
the side of the density term to help bring the left
side down to zero, to flatten the universe. So, even though the
density is still too low to reverse the expansion, with
this new player in the game, geometry is no longer tied
to the fate of the universe. A flat, k-equals-zero
universe can expand forever. But what does this
cosmological-constant thing actually do? The expression “cosmological
constant” is a clue. This term, as we commonly
interpret it, is constant. As the universe expands,
regular matter and energy get diluted away. The weird stuff described
by the cosmological constant doesn’t do that. Its density stays constant. So the bigger the universe,
the more of this energy. We call it “dark energy,”
and we interpret it as an energy possessed by empty
space itself– by the vacuum. When the universe
gets large enough, the density of regular
matter will, at some point, drop below that of
this vacuum energy, as described by the
cosmological-constant term. At that point, dark energy
will govern expansion. But when will this happen? Billions of years from now? No, it’s already happening. In fact, the universe
reached that tipping point pretty recently, on
cosmic time scales. We now live in a universe
dominated by dark energy. How do we know this? And what are the
possible consequences? Soon, we’ll get to the
second and most compelling piece of evidence
for dark energy and for the
cosmological constant. And, along with it, the
second Friedmann equation, which will give
us the insight we need to understand
dark energy’s effect on the future of spacetime. Some of the topics we’ve
been diving into lately have been pretty high-level. If any of you are looking to
brush up on your basic physics, or you’re looking for some
more great physics content, PBS Digital Studios has an
awesome new show for you. “Crash Course Physics”
is hosted by Dr. Shini Somara, a mechanical engineer
and fluid dynamicist. We’ve got a link to
it in the description. Go ahead and check it out. Now, last week we talked about
the fate of the universe. Let’s see what you had to say. OK. A number of you were curious to
know, on what scales of space is the universe
really expanding? So the universe is only
expanding on the largest scales, not at all inside atoms,
inside humans, the Earth– even inside the Milky Way. Within these regions,
the shape of spacetime is dominated by the
gravitational field of the densely packed matter. And expansion is not occurring. In fact, you need to get
millions of light years from the Milky Way for
the gravitational field of the Milky Way and
Andromeda to not dominate the shape of local spacetime. Only outside that influence does
expansion become significant. Now there are certain
models of dark energy that could have space on
smaller scales expand, resulting in the
so-called Big Rip. We’ll get to that. Mychelly Goulart
would like to know whether the density we use
in the Friedmann equation includes dark matter. Absolutely. The density that we
calculate when we figured out the fate of the universe
does include dark matter, which we can measure by
its gravitational effect in several independent ways. Even with dark
matter, the universe is just not dense
enough to recollapse. Tomek Wooff would
like to know what the job prospects for
bachelor’s or master’s graduates in physics. Hey, Tomek– great question. So actual research
jobs in physics typically require a PhD. But if you have a knack for
math and a passion for physics, that shouldn’t deter you. However, you can do pretty
well with just a bachelor’s or masters in industry. Various tech fields
have a high demand for bachelor’s or
masters physicists, like medical imaging or
radiology, energy industries, meteorology, science
education, science journalism, and definitely information
technology of all types. And, if you choose,
you can always sell your soul for the
big bucks in finance, business consulting,
patent law, et cetera. Physics graduates are hot
recruits, due to their killer problem-solving skills. And, in fact, in 2009
the National Association of Colleges and Employers
found that new graduates from physics-major programs
have starting salaries higher than any other science major. Mr. B is asking for our
outtakes of apple mishaps. Well, I’m sorry, Mr. B,
but this is a very serious science-education show. We don’t stoop to blooper humor. And, besides, we get every
take right the first time. If I throw it at 11
kilometers per second– [BONK] [THUD] –per second– [BONK] [THUD] [BONK] [THUD] Is that the good– I
mean, the up throw, there? Or– or should I be
really doing something– [CLAP] –at 11 kilometers per second– [BONK] [THUD] [THUD] MAN: Great! That was good. And we destroyed the apple. MAN: Action. Now, before we do any
general relativity– MAN: Start again– had some
eyebrow stuff doing on. That was my acting. MAN: [LAUGH] [LAUGH] [THEME MUSIC]


    there is dark energy which is the magnetic field energy density integrated over the galactic volume vacume energy supplying the required energy to keep the galaxies intact; but the dark matter has no other explanation apart from the matter inside the black holes….

    I have a question, not exactly, but maybe loosely related to this video. While working, I was pondering a semi recent video by Vsauce called "Which Way Is Down?". In this video he explains a concept that may explain gravity, and why we experience it as we do. My question is, if this does possibly explain gravity, what in this would explain acceleration per given masses involved? Is it related to the resistance to momentum chance, as in inertia, if so, what explains inertia?
    Thank you in advance if you decide to reply to this.

    The universe doesn't need dark energy, but scientists do – in order to explain the universe. Universe itself has no needs one way or another, at all, it is simply there. Some day there will be new discoveries to better describe dark energy so it's not just some mysterious force that we know nothing about – just like ether, it could even be proven non-existent if something better fits into that model.


    It could be open and closed at the same time. It could be packed into the black hole kind of object under it's horizon, where observer can't reach the edge of the universe over finite time, even though the object itself is finite… kind of.

    Dark matter, dark energy and the cosmological constant is to gravity what fibromyalgia is to diagnosis: a catch all for whatever we don't understand. I suspect we have a profound misunderstanding of "gravity" and the "gravitational time dilation" red herring, (and the extreme variability of the speed of time and its affect on mass, and the direct affect of density on the local speed of time.)

    If we comme to the conclusion that the universe expands forever, the heat death of the universe is inevitable. That leaves the universe with only dark energy aka quantum field. In the equation you demonstrate, if there is only dark energy, does it become negative ? or am I missing something ?

    Can dark energy do the same things as regular energy? Can it do work? Can it convert into heat? Does it follow the usual rules of thermodynamics? Can it convert into matter? Can it emit photons?

    Why not think of dark energy as gravity of everything pulling from the center out? Example, we are being pulled apart by the weight of galaxies way outside our cluster, even galaxies that are racing away faster than light have an effect on us in the form of gravitu. The dark energy we see is the weight of all other distant galaxies pulling us to them in all directions.

    Does anyone else want to punch this annoying guy who presents these videos a serious butt whipping?

    So if we are giving a simple example of how an Apple is thrown and gets back due to gravity…so what force is pulling back the universe back? If it's gravity than where is it's mass? Some singularity?

    Flat within 0.4 of 1% … that's not flat, that's still not flat. Flat is flat. there is no within whatever flat.

    Why must a flat space be infinite and a positive curvature space finite?
    I know that that the latter is true for surfaces in 3d embedding spaces if we want to assume that the curvature is constant but does it hold for any kind of positively curved space?
    Additionally, a cylindric surface is flat but also finite in one of its directions.

    Ya know, if space is expanding… then space can expand, which means there is some way to make it expand, which means that that 'some way' has to be accessible and manipulable. Gravitational waves show us 'space' is a thing. How do we work with it?

    Hahahaha That last part was great. XD I'm sure bloopers after every episode would get old but it was awesome to see this time. Especially the eyebrow thing. lol

    Does dark energy 'create' mass as it kind of comes into being? If so, does it 'create' it fast enough to ever reverse expansion?

    But this magical energy Lambda is in some way balancing the left side in order to "keep" the universe flat. Is it then a resisting force against curving the universe? Such as a solid ruler resisting being bent?

    Great videos. However, I suggest using a “de-easer” in the audio recordings of these videos. The sibilance from the “s” sounds is excessive and distracting from the content.

    Came back to this old vid to ask.. I was reading Neil Degrasse's Astrophysics for people in a hurry and thought around the dark matter or energy chapter… what if we just need to rethink assumptions… that's usually how weird problems get solved. what if dark matter or energy is because of neutrons or neutrinos? what if some particles have unrecognizable properties that would overall give the dark matter and energy effects that we're seeing? what if it's that new possible 5th force and maybe it's kinda like a neutrino and it doesn't really react much at all, or only likes to react in areas of space extremely devoid of gravitational bending/just gravity/mass… just thinking

    WOW – spacetime is curved (always – no matter what), but at a fixed instant in time, SPACE can be curved or flat …. thanx thats an enormous subtle constraint/freedom

    the universe cant expand for ever because of the 2nd law of thermodynamics and the conservation of energy…..

    What if sum of angles in triangle is something different but we measure it 180. We think our space is flat because w think 180 is normal but it'allready bent value

    Also. cosmoligical constat drives equation to belove zero so it should be opposite of dark energy

    I kind of regretted not watching this before. This is the best channel along with It’s Okay To Be Smart! Good Job PBS! 💪🏻👏🏻

    you said that the density of the universe is constant so when black hole shallows a star or a planet then is the total mass is conserved or not??

    what dose this mean for the speed of expiation? if we "need" dark energy & it is increasing the speed of expiation, how fast can it go & how might that effect the universe?

    My guess is dark energy is fuelled by the radiation/heat etc… from stars, possibly black holes also. however as the universe ages, and less new stars are born, the energy that fuels dark energy will decrease. at some point we will reach a point of diminishing returns of dark energy fuelled expansion and with dark energy have nothing left to power it, will begin to disappear or evaporate.

    one thing i believe and Einstein himself have mentioned could be is that space itself is not empty and is made of something maybe a mixture of dark matter and dark energy

    Sir am a mechanical engineering student from india, i like to do my higher studies on astrophysics and cosmology, which place will be the better choice ?
    Replies welcomed..

    Seems like every six months or so I read about some object or other that scientists have found are far more abundant that previously believed (e.g. brown dwarfs, neutrinos, etc.). Isn't it possible that 'dark matter' is simply the sum of all these things we are just learning about or don't yet know about that aren't really exotic at all. And wouldn't that also have an effect on the universal density portion of the field equation?

    i think we with our space ships we trying to travel through Matter .(dark Matter)it's difficult go through ,we must find way to travel through empty space where is no dark matter.or maybe make tunnel through

    So what I'm thinking is, what if our observable part of the greater universe rests very high on the y axis on the hyperbolic plane of a negatively curved spacetime, so that the curve is so incredibly slight that it falls within our margin of error for measurements? We would have no way of knowing what type of universe we live in, negative or flat.

    If entropy increase is valid in gamma rays, we are measuring the rays of super nova explosion incorrectly. That means the universe is expanding at constant speed. why there is no informational field about the entropy of the rays. for example, the entropy of cosmic microwave backround rays is increasing. Why shouldn't the entrp of the gamma rays increase and shift these rays into the red?

    If entropy is valid for gamma rays, then the universe is expanding at constant speed. in this case, we do not need dark energy.

    So time is expanding reverse exponentially in opposed to the past that eventually never happened. That essentially applies to all future possible tangents relatively temporal.

    Thanks my nightmares won’t be as bad every night I hope I well be able to be free once again from the fear the end of the universe and existence and further thinking.

    I'm really courius about one thing.
    Curvature of observable universe was masured to be 0.4% close to flat. Doesn't that mean it's NOT flat? As the observable universe is just a fragment of the whole universe, shouldn't this tiny curvature imply that on a larger scale it's not flat? It's like measuring curvature of Earth by measuring angles in triangle on a eg. 10km2 plane patch. Curvature of this patch may be around 0.4% (just guessing the numbers) what implies that Earth is round, not flat.
    If something is not perfectly flat on smaller scale, why assume that it's flat on a larger scale?
    From what I understood, this 0.4% curvature could lead to estimating Big Universe size assuming it's spherical.

    Is dark energy like potential energy? In the sense that, the higher up you move, the more potential energy you have. The further space there is between two objects, the more dark energy the objects have.The futher a galaxy is from another galaxy, the more dark energy it has. But this kind of implies in my thinking, an energy that does work. The more potential energy you have, the further gravity will pull you to the ground. Maybe it's better to instead think of dark energy, in this scenario, as negative. As in, the more distant, the less dark energy the galaxies have. They tend towards the easier state, the one with less energy. It requires more dark energy for galaxies to be closer together. And then in that case, the negative sign in front of the dark energy portion of the equation is coming from the dark energy to begin with.I know this is a misunderstanding of what I'm beginning to be taught about dark energy, but that was the immediate relation I made in my mind. I think I need a better understanding of why dark energy needs to be subtracted from the rate of expansion in the equation and watch the video on cosmic inflation.

    Youtube comments are full of amusements sometimes….

    Dark Energy is the gravitational potential energy we gain as space expands away from the White Hole that is the big bang.

    When one regards the infinite universe as one big quantum field with different states within it, your perception completely changes. Many physical properties become insignificant. We are a quantum state. Time is also not relevant anymore…. It's always now, past and future disappear.

    Is time related to the shape of the Universe? And if so, how does it act in a curved Universe, for example?

    Actually you are doing very good. I think you are climbing towards the top, or maybe already are there. Clever, funny and charismatic. And just to clarified I’m straight = sexual orientation. But thumbs up 👍🏼. See you at the Nobel committee. A price, good gourmet food and apparently some horny people. So if you ever heard someone saying this is so boring. Then they haven’t been there. Reality always seems to beat imagination. Wow 🤔🍀🐈🧪🎸

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