Dark Matter and Dark Energy: The Frontier of Astronomy

By | December 6, 2019


Hey, it’s Professor Dave, I wanna tell you
about dark matter. When you really think about everything we
have discussed in this astronomy series, you have to admit that our current understanding
of the universe is nothing short of astounding. All of those objects in the night sky that
we’ve wondered about since we could talk, they are no longer gods and demons. They are real, physical objects, with properties
and characteristics that we can measure, categorize, and predict with incredible accuracy. We know about planets, asteroids, comets,
stars, black holes, and galaxies. We know what they’re made of. We know how they behave. We’ve sent objects through space, put men
on the moon, and built sophisticated telescopes that can see nearly to the edge of the observable
universe, giving us a solid picture of how and when the entire universe began. But even with all this, we don’t know everything. Just as with any other field of science, there
are areas of active study, with brilliant minds all over the world trying to answer
perplexing questions about the universe. One of the most puzzling of these areas regards
our understanding of matter and energy. Over the past few decades, it has become increasingly
clear that there must be some kind of matter out there in space that we can’t see. This isn’t as mysterious as it sounds, as
long as we remember what it means to see something. An object emits electromagnetic radiation
when electrons in its atoms relax from some excited state down to some lower energy state,
generating photons in the process. When those photons hit our eyes, we see the
object. So if something is not capable of producing
photons in this manner, or related nuclear processes, it can’t produce visible light,
or any other kind of light that we can detect, like UV rays, radio waves, microwaves, or
any of the rest. But the particles that make up atoms aren’t
the only kinds of particles there are. There are lots of different kinds of particles,
so it’s not outrageous to think that there could be macroscopic quantities of matter
that don’t interact with light. Because of this behavior, we assigned it the
name dark matter. And although we don’t know what it is, we
know what it’s not. It can’t be any kind of baryonic matter,
which is all the protons and neutrons that make up all the ordinary stuff we can see. How much dark matter is there? A lot, apparently. Even though we can’t see it, it still exerts
gravitational influence, so we can measure the mass of some amount of dark matter by
analyzing the motion of nearby objects. The best example of this has to do with the
orbital velocities of stars within a galaxy. Stars should travel at a speed dictated by
how much mass is in between them and the center of the galaxy. So stars very close will go a little slower,
as there isn’t as much mass in there. Moving farther out, there is more mass tugging
a star inward, and they should orbit faster. Going towards the edge, stars should slow
back down a bit, as despite the additional mass, they are getting very far away, and
gravity weakens rapidly with distance. So we should expect velocity to rise rapidly
and then decrease as we move from the center of the galaxy to the edge. But in fact, this is not what we observe. Velocities rise initially as expected, but
at a certain distance they level out, maintaining roughly the same velocity all the way to the
edge, violating Kepler’s second law. This seemed puzzling, and it was proposed
that there must be large amounts of unseen mass in the outer regions of a galaxy, including
ours. After doing the math, it was realized that
there actually must be up to ten times more dark matter than regular matter, with every
galaxy being embedded in a dark matter halo. By making this assumption, the behavior of
all the galaxies began to make perfect sense. Now you may be wondering how we can jump to
such a nonsensical conclusion, rather than just restructuring our notion of orbital velocities. Beyond the certainty of the math, there are
other ways of demonstrating that dark matter exists. When we learned about general relativity,
we saw that matter warps spacetime, and if light travels through warped spacetime, its
trajectory will be affected. This is observed through a phenomenon called
gravitational lensing, where the light from a distant object curves around a nearer object,
sometimes in more than one direction, making several copies of the object in the sky, or
fanning the light source out into smaller regions of light. When this nearer object is something visible,
like a cluster of galaxies, we can perform important calculations from such an observation,
and calculations show that the magnitude of the lensing requires much more mass than appears
to be in the cluster. Beyond this, gravitational lensing is sometimes
witnessed around seemingly empty space. In addition, when we look at the structure
of the observable universe, galaxies seem to be gathered into filaments, with huge voids
in between containing almost no galaxies, as though some network of massive objects
is responsible for directing them there, and calculations suggest that the amount of luminous
matter we see could not have been enough to cause matter to initially accumulate into
stars and galaxies in the time that they did. The notion that cold dark matter, unaffected
by radiation, may have experienced perturbations in its uniform density first, accumulating
to form large-scale structures with a gravitational potential that luminous matter was then attracted
to later, perfectly accounts for a huge amount of observation. So between galactic orbital velocities, gravitational
lensing, and a wide variety of other data regarding the evolution of galaxies and other
such phenomena, dark matter seems to be here to stay. So what are the best candidates for dark matter? We know it’s non-baryonic, so what does
that leave us with? There could exist subatomic particles that
go beyond the standard model of particle physics, which could account for these observations. One such type are called WIMPs, or weakly-interacting
massive particles. It may even be some kind of neutrino, supersymmetric
particles, or any number of other exotic particles that have been postulated but unconfirmed
by experiment. So this is a prime example of a situation
where astronomers and particle physicists have to work together to solve a problem. Unfortunately dark matter is not the only
burning question in astronomy. There appears to be a similarly mysterious
agent at work, which we call dark energy. This concept arose as a result of measuring
the rate of expansion of the universe. We had thought that because of the gravitational
attraction between all matter, the rate of expansion should be slowing over time, as
all the galaxies tug on each other, potentially even halting the expansion and eventually
contracting back into a single point. This event would be called the Big Crunch,
essentially the precise opposite of the Big Bang. However, observations show that this is not happening. Instead, the rate of expansion is actually accelerating. What could possibly be causing this acceleration? Gravity is only attractive, but is there some
repulsive counterpart that becomes apparent at very large scales? Is it some other thing totally unrelated to gravity? We don’t know, so we have dubbed this phenomenon
dark energy. Do we have any ideas as to what it could be? We do. One of them brings back Einstein’s cosmological
constant, his self-professed biggest mistake. It turns out that it might not have been such
a mistake after all. Einstein initially invoked this concept to
support the steady state model for the universe, describing a constant energy density that
fills up every point in space evenly. In his model, it was necessary to counteract
any expansion or contraction to yield a static universe. But this concept could just as readily apply
to this problem, with such an energy that actually accelerates expansion. We represent this constant with a capital
lambda, and we can think of it as a kind of vacuum energy, the energy contained within
space itself. It’s extremely weak, and was completely
masked in the earlier stages of the universe when matter was more dense. But as everything continued to drift apart,
the universe becoming more sparse, and the effects of gravity weakening, lambda continued
to occupy every cubic centimeter of empty space. Around five billion years ago the universe
passed below a density beyond which lambda began to be significant, pushing everything
apart even faster, and today we see a universe expanding at an accelerating rate. But all of this is even more hypothetical
than what we have already said about dark matter. We really don’t know what dark energy is
or how it does what it does. There are competing models, all of which are
based on some set of observations, but remain largely speculative. So to put things into perspective, based on
present calculations, we believe that dark energy makes up about 68 percent of all the
energy in the observable universe. Dark matter makes up about 27 percent, by
mass-energy equivalence, while only the remaining five percent is represented by all the stuff
we can see. Even still, while we must admit that there
are plenty of things about the universe that we don’t understand, there are also plenty
of things that we do. Enough even to understand how the universe
began, so it’s not that surprising that we can propose hypotheses regarding how the
universe will end. Let’s move forward and see what some of
these possibilities could be.

27 thoughts on “Dark Matter and Dark Energy: The Frontier of Astronomy

  1. Gautam Chettiar Post author

    You can create an institute where youre the physics, chemistry, maths, biology, Italian, English, astrophysics, astronomy and quantam physics teacher.

    Reply
  2. constellationpegasus Post author

    Again, thanks for the great explanations that tie these different subjects in each video.

    Reply
  3. Giorgio Giorgio Post author

    At 1.57 you say "if something is not capable of producing photons in this manner, or relate nuclear processes, it can't produce visible light". As far as I know, all electromagnetic radiations (visible or not visible to our eyes) consist of photons emitted by the same process you have described for visible light, that is: electrons moving from excited states to lower states.

    Reply
  4. Q&science Post author

    I am very troubled by this: people seem to refer to dark energy as vacuum energy, the energy contained in vacuum due to quantum fluctuations. But I heard that if you calculate the quantum energy and compare it with the observed magnitude of dark energy, the numbers are wrong by like 20 orders of magnitude.

    Reply
  5. Terry Miller Post author

    In our solar system, the planets closest to the sun move faster in their orbit than those farther from the sun.  Why is this different in the galaxy you used in your example?

    Reply
  6. Mohammed Abdullah Post author

    Great video, do you think you may be able to make a video on some of the concepts in m-theory

    Reply
  7. The Goodly Dragon Post author

    Thanks for clarifying. I had thought previously that it was called dark because it wasn’t yet understood. I think it’s interesting that it’s dark in the literal sense that it doesn’t emit light.

    Reply
  8. The HoSS Post author

    All the problems you are stating about the standard model is characteristic of electrical activity. No need for more particles or increasing complexity.

    Reply
  9. M Praneeth Post author

    Dark matter can't be seen but can dark matter obstruct the path of visible matter?

    Reply
  10. Rajesh J Post author

    It could be that the fundamental unit of energy which is 'Prana' of the physical observable universe, be the source of dark energy filling in all atoms, and the spaces in between.

    Reply
  11. Bill Fauber Post author

    Wait you haven't been keeping up with current news. We were all taught that 80% of the universe is made up of Dark Energy and Dark Matter. In the past 6 months there has been no less than 20 papers released which indicate that all the Dark Matter/Energy models maybe incorrect. Billions upon Billions of dollars spent on detectors and no one has found anything. The top physicists on our planet are now saying they may have been wrong. Have you heard any of this in the Mainstream News Media "NO" and why is that, it's because it's a major paradigm shift in critical thinking and academia is fighting these new findings because it destroys their perfect world assumptions. Imagine what we would know if money, pride, conceit, arrogance, snobbishness, politics and religion were removed from the new advancements in science and history, oh what we could learn.

    Reply
  12. Farshid Azar Post author

    In this playlist, the next 5 videos (27-31) show as [private video] and do not play. Is there a reason for this?

    Reply
  13. BigNewGames Post author

    Models trying to describe dark matter do not support dwarf galaxies on a near flat plane "organized" surrounding larger galaxies. Dwarf galaxies should be erratically positioned. Models also determined the dark matter should be concentrated in the center of galaxies and that is clearly wrong too.

    Reply
  14. KilBu Post author

    Super interesting video! thank you for sharing <3

    Reply
  15. michaelbauck Post author

    My idea about "Dark Matter " is that it's a sort of background gravity field, caused by the expansion of the universe.
    I can't get the idea out of my head, that something that is expanding on such a scale and speed as the universe is, wouldn't experience some sort of warping effect on it's own fabric.
    I would love to explore this idea formulaically, but I am a total
    "mathaphobe" and wouldn't know where to start,so I keep putting this idea out there hoping someone smarter than me will latch on to it.

    Reply
  16. nmout Post author

    So expanding space is automatically filled with newly produced out of nothing dark energy ?

    Reply

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