Hi there, physics fans. We’re moving on in our exploration of the some of the mysteries
of modern physics. I’ve talked about dark matter, but that’s only one cosmic conundrum.
There’s also dark energy, a phenomenon with a similar name and a totally different behavior.
Dark energy will determine the fate of the entire universe, making it an ideal topic
for this week’s episode of Subatomic Stories.
In earlier episodes, I spent a great deal of time talking about general relativity and
gravity in general. That’s because gravity is the force that rules the cosmos. The other
known forces make stars shine, but it’s gravity that tells them where to go.
And gravity has been ruling the heavens since the beginning. The Big Bang caused the universe
to expand rapidly, leading to galaxies moving apart from one another for nearly 14 billion
years. Since familiar gravity is an attractive force, during all those years, gravity has
been slowing down the expansion of the universe. The question, however, is what will win? The
expansion or gravity?
If you throw a ball upward, it can fall back down, go high enough to support and just orbit
the Earth, or escape the Earth and fly away into deep space, the same thing is true of
the universe. Big Bang theory says that when the universe began, matter was flung apart
from one another with a specific speed. So, given this initial speed, the question becomes
“what will happen to the mass that makes up the universe?”
Will the universe expand for a while and then collapse back in a Big Crunch? Will it expand,
always slowing, and finally stopping at time equals infinity? Or will the universe expand,
always slowing, but never stopping? Even a basic understanding of gravity says that there
are only these three options.
So, in 1998 a couple groups of astronomers decided to figure out what was truth is. What
is the fate of the cosmos? They did so by looking at distant galaxies. They figured
out how far away they are by looking for a particular type of supernova, which has a
known brightness. By seeing how bright the supernova was in their telescopes, they could
determine its distance.
They also measured the velocity of the galaxy by looking at its color and seeing the difference
between the emission lines of hydrogen in those galaxies, compared to the light emitted
by hydrogen in a stationary laboratory on Earth. The light from distant galaxies is
redder than we see in our terrestrial experiments and, from the amount of change of color, we
can work out the speed of the galaxy.
It sounds complicated, but it’s the same effect as the change in pitch you hear of
the engine of a race car as it drives by you, changing from coming towards you to moving
away. So that’s the basic experiment – figure
out the speed of distant galaxies as a function of distance. Each of the three possible scenarios
have a different and distinct velocity profile. So, it should be easy to figure out which
scenario is true. Just how much was the universe slowing down? Was it one, two, or three?
You know…there’s a reason we don’t just do theoretical physics and go out and do real
experiments. And that reason is because sometimes nature can surprise the heck out of us. Because,
when we asked which of the three scenarios described our universe, the answer was a jaw
dropping four.
In spite of the fact that all three scenarios predicted that the expansion of the universe
was slowing, the measurement quite clearly showed that it was speeding up. Or, more accurately,
the universe was slowing down for the first nine or so billion years of its history but,
about five billion years ago, the rate of expansion turned around and has been getting
bigger and bigger that whole time. That’s slightly longer than the Earth has been around.
So, if gravity is an attractive force, it stands to reason that this observation is
impossible. Accordingly, we can only conclude that gravity is a repulsive force, which flies
completely in the face of common sense. We know that’s not true.
Well, let’s not be hasty. Einstein invented general relativity back in 1915 and his equations
rather clearly said that gravity is really the distortion of space and that the correct
view of gravity is that matter and energy distorts space and time. We talked about this
over lots of earlier episodes and it’s embodied in this impressive looking equation here.
Even in the early days, Einstein was aware that general relativity clearly predicted
that space was changing over time and the theoretical prejudice of that era was that
space is eternal and unchanging. So, in order to balance the tendency of gravity to pull
matter together, he added a term to his equation, which included a repulsive form of gravity.
This term is called the cosmological constant. The attractive and repulsive terms cancelled
out, predicting that the universe was stable.
But, of course, in 1929, Edwin Hubble found that the universe was expanding due to the
Big Bang. Basically, Einstein said “oops,” or however they say that in German, and took
the cosmological constant out.
However, with the discovery of the accelerating expansion of the universe, general relativity
needs a substance that generates a repulsive form of gravity. Astronomers call this substance
“dark energy,” and it is an energy field that permeates the entire cosmos.
Now, scientists are not entirely sure what dark energy is, but the cosmological constant
is one possible explanation. The cosmological constant is a constant energy density in the
universe. And, yes, that means that as the volume of the universe increases, dark energy
also increases. That might seem like it’s a problem, but as I described in episode 20,
general relativity predicts that in a changing universe, energy isn’t conserved. So, it’s
not the problem you’d think.
There’s another model of dark energy called quintessence. Quintessence is like the cosmological
constant, but it changes over time. And, if dark energy changes over time, it has profound
consequences on the fate of the universe. So, it’s important to figure this out. Currently,
the data is consistent with a constant density of dark energy, but future experiments may
change that. Maybe I’ll deal with this in a future video.Okay, so that’s a lot, but
dark energy is a big deal. But, then again, so are questions. Let’s see what questions
our viewers have for us this week.
Questions…there were tons of dark matter questions. Let’s get started. Jonathan Hacker
asks about gravitational lensing by intergalactic dark matter. Hi Jonathan. Short answer – yes.
There is a lot of evidence. Take for example this picture of galaxy cluster Abell 2218
– a cluster containing about 10,000 galaxies. The yellower galaxies are relatively nearby,
only about two billion lightyears away. And yes, I use the word nearby ironically.
The orange and blue arcs are distorted images of even more distant galaxies. One of the
galaxies is as far as 13 billion lightyears away. Astronomers added up the mass in the
visible parts of Abell 2218 and it just isn’t enough to account for the distortions. The
only way to see what we see is if there is about 25 times as much dark matter as ordinary
matter in the cluster. So, there’s your evidence.
Richtalk34 asks if the final evidence for dark matter will come from an astronomical
or particle physics break through. Hi Richtalk. That’s easy. It’ll be particle physics.
There are three approaches to seeing dark matter – making it in accelerators, seeing
dark matter interact with matter as it passes through the Earth, and seeing dark matter
and antimatter dark matter annihilate in locations like the center of the Milky Way.
The only way anyone will really believe that dark matter has been discovered is by seeing
in underground detectors dark matter passing through the solar system, combined with creating
it in particle accelerators. Seeing the annihilation products from elsewhere in the universe is
far harder to prove. It’s too easy to generate similar signals by commonplace things like
pulsars and things like that. Nope – we’ll see it here on Earth, or we’ll never believe
it.
Xarniaa ask if the existence of galaxies without dark matter implies the existence of galaxies
with only dark matter. Hi Xarniaa. The answer appears to be yes, or at least mostly so.
Astronomers have reported Dragonfly 44, a candidate galaxy with the right properties.
It reportedly has a mass about the same as the Milky Way, but it emits about 1% of the
light the Milky Way.
Furthermore, some astronomers claim that dwarf galaxies have a much higher fraction of dark
matter than bigger galaxies. I should point out that there is a lot of controversy on
this topic in the community. There seems some evidence for the existence of dark matter
galaxies, but I might say something different next year. That’s how science works on the
frontier of knowledge. Welcome to my world.
Rohin S asks how dark matter formed and how did it interact with matter in the early universe.
Hi Rohin. That answer is easy. I dunno. Remember that the experimental status is that dark
matter hasn’t been inarguably confirmed and we have no knowledge of the properties
of dark matter particles. Until that happens, any answer to your question is nothing more
than informed speculation.
Max Ryan asks if a degree in theoretical physics is a good choice for a person wanting to become
a theoretical physics. Hi Max. Well…yeah. I mean, I suppose it’s possible that someone
can do it on their own, but the odds of success are essentially zero. The myth of an errant
genius is just that – a myth. I should warn you, however, that a degree doesn’t guarantee
a career.
The number of theoretical physicist positions that become available at research institutions
is maybe 2 or 3 per year in the entire US, including all branches of physics. And theorists
are all very smart, so the competition is intense. There are many more theoretical faculty
positions at smaller, teaching-oriented, colleges. But, even then, the other applicants have
stellar credentials and brilliant minds. In short, a career in physics theory is a very
hard path. You should have a plan B. Luckily, many industries have problems that need well
trained and smart people.
T J Reynolds asks if the Milky Way has any dark matter. Hi TJ. Yes. We don’t have a
super precise measurement, but astronomers think that the Milky Way has, ballpark, 10
– 20 times more dark matter than ordinary matter. In the vicinity of Earth, we believe
that DM makes up about one proton mass per 3 cubic centimeters. That works out to being
equivalent to one asteroid’s worth of dark matter in the entire solar system.
However, astronomers believe that the dark matter is not in a clump, but rather is dispersed
throughout the solar system. They have underground detectors searching for the dark matter wind
blowing through Earth. So far, no luck. But we keep looking.
Psychx asks if dark matter could be micro black holes. Hi Psychx. In principle, the
answer is yes, but we’ve looked, so the real answer is no. Searches for things like
black holes have ruled out objects in the mass range of a third the mass of the moon
to a hundred times the mass of the sun. Bigger and smaller are still possible. Given the
amount of dark matter in the Milky Way, we can be pretty sure that it’s not the form
of big black holes, or we’d have seen them by now.
Now, smaller black holes are not excluded, but it’s hard to imagine how they could
be made. Personally, I think this is an unlikely answer to the dark matter question. But it
can’t be ruled out with 100% certainty.
Sunsparkda commented on the controversy surrounding the announcement of galaxies with no dark
matter. Hi sunsparkda. You’re right, but things are moving fast. In 2018, a group claimed
to have found the dwarf galaxy NGC1052-DF2, which has no dark matter. The following year,
another group said “you’re nuts. You got the distance wrong.”
The first group then replied “No way. We’re not nuts. We made some additional measurements
that validate our first paper. Oh, and here’s another galaxy just like the first one.”
And then another group claimed to have found 19 more small galaxies without dark matter.
And so it goes. This is happening right now, and the situation is evolving rapidly. However,
the scale seems to have tipped in the direction of the observation of galaxies without dark
matter being true. But the pendulum may swing the other way next week. We’ll have to wait
and see.
Jose Paulo asks why you need EM for dark matter to clump. Hi Jose. I guess, technically, you
don’t. But you do need a force that causes matter to stick together. Gravity can pull
objects towards one another but, even if they collide, energy must be conserved. So the
objects just bounce off one another. To assemble into a bigger object, they have to stick together.
That means electromagnetism or something similar.
Giuseppe Tumino notes that dark matter could have the same fate as the luminiferous aether.
Hi Giuseppe. Yes. Yes, it could.
Okay, on a different topic, I’d like to remind you that I’ll be giving an online
public talk a couple of days from now via Zoom. It’s called “Understanding God’s
Thoughts: Einstein’s Unfinished Dream” and it’s about progress towards a theory
of everything. I’ll also take some questions. If you want to attend, you have to register,
and I put a link in the description where you can do just that. And, if you miss the
lecture, we’ll post it as a video on this channel a week or so later. I hope to see
you there. Okay, so that’s all the time we have today.
I hope you enjoyed it. If you did, please like, subscribe, and share. And, if you didn’t,
why not? How could you not? I mean…who doesn’t love physics? After all, even at home, physics
is everything.