There is no doubt that Isaac Newton was one of the greatest scientists that ever lived.
He united our observational experience of gravity on earth to the motion of the stars.
He showed that the gravity, experienced by a falling apple, can be described by exactly
the same equation as the gravity experienced by earth going around the sun.
Newton’s equation was considered gospel for hundreds of years.
It was noticed however, that Newton’s equation did not quite predict the observed motion
of the planet Mercury, the planet closest to the sun.
Mercury has something called a precession – this means that its elliptical
orbit around the sun varies slightly every time it goes around the sun. And this shift
in orbit was much more severe than predicted by Newton’s laws. No one could explain it.
In 1859, French Mathematician Urbain Le Verrier proposed that this odd motion of Mercury could
be explained by a kind of dark matter. This dark matter, he proposed, was an unseen small
planet , called Vulcan, that lay somewhere between Mercury’s orbit and the sun.
Of course, no one could find such a planet. But scientists explained that this planet
was obscured because the sun was so bright that anything small in front or back of it
could not be seen. This theory held until the 1920’s when Einstein’s theory of general
relativity more precisely described the laws of gravity. And it perfectly predicted the
observed precession of Mercury due to the curvature of space time. So the dark matter
called Vulcan turned out to really be due to an error in Newton’s laws.
By the same token, is it possible that what we have invented as dark matter, to explain
the motion of stars in galaxies and clusters, is like Le Verrier’s planet Vulcan - a figment
of our imagination? Could it be that Einstein’s General Relativity has a flaw?
This is a serious school of thought among some scientists, that has surprising evidence
to support it. The details are coming up right now...
As I showed in the previous video, Vera Rubin in the 1970s, verified that stars in the outer
edges galaxies are moving around the galactic center at about the same speed as those closer
to the center. But according to the laws of gravity they should be moving much slower
because the effect of gravity goes down by the square of the distance.
This can be seen in the motion of earth and all the planets in our solar system.
The planets further away from the sun are moving much slower than planets closer to the sun.
But for some reason, this inverse square law does not appear to work based on the mass
of all the visible matter in galaxies. The motion of stars for example in the Milky Way
galaxy is about 200 km/second regardless of its distance from the center.
You can see how this occurs on this computer simulation. The left shows how the galaxy
should behave based on all the visible matter. The galaxy simulation on the right is how
the galaxy actually behaves. Note how the motion of stars on the outer edges are much faster
than they should be.
This faster than predicted motion, it was theorized, is due to the extra unseen matter, or Dark Matter
that surrounds all galaxies. And the amount of dark matter needed to make this galaxy
behave the way it does is about 6 times more than the observable matter. So this dark matter
makes up about 85% of all matter, and 15% is the visible matter that we can see.
But in decades of research, no one so far, has detected a dark matter particle. Could
it be, like the planet Vulcan, that it actually just doesn’t exist? And could this motion
be equally well explained by a modification of Einstein’s General Relativity?
General relativity has been shown to be highly accurate for massive objects that are very
short distance apart up to several light years. But when it comes to the galaxy however,
we are looking at a radius of 50,000 light years, and clusters that are millions of light
years across. Is it possible that gravity behaves very differently, at thousands of
light years across than it does at say, 1 light year across?
Is there a better, more accurate theory than General Relativity? In 1983, Israeli physicist
Mordehai Milgrom, published a hypothesis showing that a modification of Newtonian mechanics
could almost perfectly explain our observations.
Milgrom noticed that in the centers of galaxies, no dark matter needs to be invoked because
there is enough ordinary matter to explain the motions of stars. Only in the outskirts
is the motion so fast, that we need to invoke dark matter to explain the data.
What Milgrom found was that if you calculate using just the laws of Isaac Newton, the acceleration
due to gravity at the point where ordinary matter is no longer sufficient, where you
need dark matter to explain the velocity of stars, this acceleration was the same for
every galaxy.
In other words, there was some critical value of acceleration, such that if the acceleration
was larger than that value you didn’t need dark matter, but when the acceleration was
smaller, you did need dark matter.
So what Milgrom said is that maybe there isn’t any dark matter, maybe we just need to modify
the theory of gravity such that if the acceleration is greater than a certain value, it’s a
one over radius squared, as Newton’s equations indicate.
But, at smaller accelerations, gravity has a force that is greater than that. Maybe it
falls off at one over the radius, instead of radius squared.
Now, it may have sounded crazy to make Newton's laws change this way, but
the remarkable thing is, that if you invoke Milgrom’s hypothesized modification of gravity,
it fits the data like a glove, almost perfectly! It fits our observations of the motions of
stars not just in our own galaxy, but dozens of other galaxies. The (red) lines here are the
predicted values, and the (blue) points with the error bars are the actual measured data points.
So the idea that you don’t need dark matter, and Newton’s laws work just fine inside
a certain radius, but a modification of Newton’s laws works on the outside of this radius, seems
to make it work quite well in galaxy after galaxy. Milgrom’s hypothesis is called Modified
Newtonian Dynamics, or MOND. And as you can see from the data, it cannot be ignored.
Both the idea of MOND, and the theory of a dark matter halo surrounding galaxies seem
to work equally well. But since we haven’t detected dark matter in decades of looking
for it, why don’t we just adopt Milgrom’s hypothesis as a new scientific theory, and
get rid of the idea of dark matter as a particle altogether? Is there any problem with Milgrom’s theory?
Well, it turns out there is. And as you know, there are very few things that work perfectly
in science.
The problem is that while Milgroms theory explains the motion of stars within galaxies
exceptionally well, it does not explain the motion of clusters of galaxies.
This means that the bullet cluster that I talked about in the previous video is not
explained by Milgrom’s theory. In this cluster, the pink area is where ordinary matter, in
the form of interstellar gas, reacts with itself, and can be seen as X-rays. But gravitational
lensing indicates that the majority of the mass of this cluster is located in the blue
areas, where less ordinary matter can be seen. So there must be unseen dark matter in these
areas as indicated by the gravitational lensing.
This is very strong evidence for the idea of dark matter being a particle. This cannot
be explained or predicted by Milgrom’s modifications of newton’s equations. And many scientists
have tried very hard to make MOND work in these cases, but have been unsuccessful. So
most proponents of MOND today believe that you cannot get rid of the idea of
a dark matter particle altogether.
So maybe both ideas are correct. And maybe we need both - a modification of Newton’s
laws, as well as dark matter particles to fully explain our observations of both the motions
of stars within galaxies on scales of tens of thousands of light years, and the motions
of clusters of galaxies millions and billions of light years across.
One benefit of adopting both ideas would be that it would allow the possibility that the
dark matter you would need to explain the behavior of clusters of galaxies
may be different. It may be ordinary matter, rather than
the popular candidates of dark matter, namely WIMPs and Axions, which are theoretical
and have actually never been detected.
This dark matter could perhaps be ordinary matter such as Neutrinos. As I said in my
previous dark matter video, scientists don’t think neutrinos can explain the motion of
stars in galaxies because neutrinos are hot, meaning they move too fast to allow halo formations
around galaxies. But they do move slowly enough that they could form halos around clusters
of galaxies, on the scales millions of light years across, such as the bullet cluster.
So proponents of MOND now believe that motions within galaxies can be explained by a modification
of Newtonian gravity, and the behavior of clusters of galaxies can be explained by some
type of ordinary matter such as neutrinos, or as yet unseen dark matter,
like Wimps and Axions. So
while it was the dream of many physicists and MOND proponents to get rid of the idea
of dark matter altogether, I think, it is very difficult, if not impossible.
The bullet cluster for one makes perfect sense if there is dark matter, but makes no sense
if there is only ordinary matter.
But another nagging reason is the microwave background radiation. This is the way the
universe looked at about 380 thousand years after the big bang, about 13.799 BILLION years
ago. And its structure tells us a lot about the way ordinary matter came together to form
protons and neutrons, in a process called nucleosynthesis. And based on matching our
observations with predictions made by known equations, we can estimate that the amount
of ordinary matter in the universe, and it's 5%. But the critical amount of matter needed
to make a flat universe, that is a universe that is not curved in 4 dimensions, like our
observations indicate, is 30%. So this means that 25% of this 30% can not be ordinary matter.
This number matches perfectly with our observations of galaxy rotations. So because our observations
show that the maximum amount of normal protons and neutrons are not more than 5%, the remaining
25% has to be a different kind of particle, and not an ordinary particle. And this fact
is really important to understand, if you want to buy into the idea of MOND vs dark matter particle theories.
There are many things that we don’t know, as I said in a previous video, there are unknown
unknowns, meaning things that we don’t even know that we don’t know. But dark matter
is not one of them. Dark matter is a known unknown, meaning it is something that is known
to exist, but we just haven’t found it yet.
But given the number of people working on it,
and the extraordinary efforts being made to find it, I feel that we are on the cusp
of discovering it. And I’m glad that I live at a time in human history, when we will
make this kind of momentous discovery.
If you haven't already, be sure to check out my previous
video on what kind of particles dark matter could be.
And let me know what you think about dark matter in the comments below.
I'll see you in the next video my friend!