If you’re going down the highway and your speedometer is saying 80 km/h,
how fast are you moving? Before you accuse me of
asking a stupid question, hear me out. What’s movement? To make any statement
about movement we have to define reference points. You might be sitting on a train going 120 km/h,
but this speed is in relation to what’s outside the train.
Relative to everything inside, nothing is moving. This is the same for the universe. You might be
sitting still relative to the device you are watching this video on, but you are moving
incredibly fast, because the earth is moving. The earth orbits the sun
at 30 km/s! This is even faster than the rockets we use to send astronauts into space with.
But our solar system with the sun at its center is just one of many in our Milky Way galaxy.
In the center of our galaxy is a supermassive black hole,
and the stars of the solar system are orbiting it. This means that actually
you and I are also hurtling around this black hole at around 230 km/s.
Now you might think this is it. But it’s not it. Our galaxy is not standing still either,
it’s attracted by another massive object in space called the great attractor. Our galaxy is hurtling
towards this point at move than 600km/s. What is this mysterious great attractor? Why
are we heading towards it? And what does this all mean for our future? That’s coming up right now!
Before talking about the great attractor, I want to give a big shoutout to MagellanTV.
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So I thank you for that. Now back to the show. At the scale of the universe there is one force
that dominates and that is the force of gravity. Gravity keeps the earth in orbit around the sun,
which is turn is kept in orbit around the center of the galaxy.
Our galaxy is part of an enormous collection of more than a million galaxies called the Virgo
super cluster. It’s stretches across more than 100 million light years.
The gravity of all the mass in this super cluster is responsible for the keeping it together.
So, in short, the movements of these large-scale structures are due to gravitational attraction.
And gravity comes from massive objects within these structures.
The more mass there is, the higher the attraction.
But gravity diminishes as the distance between massive objects increases.
This can all be understood from Newton’s law of universal gravitation:
Where F is the force of attraction between two massive objects m1 and m2,
that are a given distance r apart. G is the gravitational constant.
It’s like a fixed ratio relating the masses with distance between the masses. So we clearly see
the relationship between mass and distance. Shorter distance and more mass means a stronger
gravitational attraction. Longer distance and less mass means weaker gravitational attraction.
This is fairly simple. But what is all the fuss about the great attractor?
As with many things, the mystery developed over the years as our instruments
could see further out into space, and we learned more about the universe.
Remember, until recently, we thought our Milky Way galaxy was the entire universe.
We didn’t know there were a myriad of other galaxies until Edwin Hubble came along in in 1923.
Actually 18th century the French astronomer Charles Messier discovered the first galaxies,
but he didn’t know it at the time. He simply cataloged them as celestial objects.
It was another 200 years before Hubble figured out what these fuzzy objects were.
And since then, we’ve found countless other galaxies and realized that they form larger
clusters creating a kind of filament consisting of walls of gravitationally
bound galaxy superclusters, many of which are billions of light years in size.
And over time, gravity has caused these galaxies and clusters to group into large
web like structures containing not only visible matter, but also huge voids where
there appears to be almost nothing visible. For a while we thought that these large
structures were stationary, but in the 1970’s we began getting solid evidence that our galaxy
is not stationary, but is indeed moving through space. Initially scientists concluded that we were
moving towards the constellation of Centaurus. In the 1980’s the mystery deepened
as we discovered that not only the Milky way, but approximately 400 other galaxies were also moving
towards a certain point in space, not quite towards the constellation of Centaurus.
Something was attracting our Milky Way and hundreds of other galaxies towards it.
That something was elegantly named the “Great Attractor.”
But what is the great attractor? It was calculated that since it had the attractive power pull so
many hundreds of galaxies, it must have a mass of more than 10 million billion suns.
How did we come up with this number? This can be approximately calculated using simple
Newtonian physics, by the same equation I showed earlier, solving for M2.
More evidence for the attraction comes from the cosmic microwave background, or CMB.
The CMB is the leftover background microwave radiation from shortly after the big bang.
It turns out that picture is affected by how we move in space. How you might ask?
As our galaxy is hurtling towards the great attractor it is also moving with respect to
CMB which acts like an unmoving background. And we're moving to the tune of 627 km/s. So the light we receive
from the CMB is distorted due to a doppler effect, analogous to the way sound changes
to a higher pitch as an ambulance approaches you and then to a lower pitch as it drives by.
The light we see in front of our direction of motion is slightly more blueish because
the wavelength gets contracted, and the light behind us becomes more reddish.
And this is precisely what can be seen in what’s called the cosmic microwave background dipole.
This dipole of slightly different frequencies of light in the CMB, is a
result of our Galaxy moving with respect to the CMB towards the Great Attractor,
along with our Local Group of galaxies. And with the help of this CMB dipole we can
roughly locate the source of this great attractor. So, you might think, aha,
mystery solved. We know the direction, all we have to do is just look and check what’s there!
Not so quick because it turns out we can’t quite do that because something is blocking our view.
What is that something? It’s our own Milky Way galaxy. The stars and interstellar dust in the galaxy
obscure our view through the galactic plane. The section of space where the Milky Way is
blocking our view of the wider universe is called the Zone of Avoidance.
Of course, we could wait until our solar system has rotated to the other side of the galaxy to
get a clean view, but it takes our sun at least 225 million years to make one full rotation around
the galaxy, so we would have to wait more than 100 million years for this view. Who knows if
we will even exist that far into the future? Luckily scientist don’t give up that easily.
While it is hopeless to detect light from the visible spectrum of electromagnetic radiation,
radio and infrared light can penetrate through the obstruction and give us
a vague picture of what is behind this zone of avoidance.
But the big help has been new technology that allows us to look at X-ray signals.
It turns out superclusters emit x-ray light. So, if the great attractor is made from one or more
superclusters, it should be possible to see the X-ray contour of this mysterious great attractor.
In 1990, this was made possible in with the launch of the ROSAT X-ray satellite. And in 1996,
Scientists published their findings. What they found was a rich cluster of galaxies
in the center of the great attractor, called the Norma cluster, which has an estimated mass of
about 10^15 solar masses or 1 quadrillion suns. Now again you might say, great mystery solved.
Well, not quite, because you see, even though a quadrillion suns is a lot of mass,
it is not nearly enough to explain the movement of the hundreds of galaxies that we observe.
There was simply not enough mass in there. So a systematic X-ray search was started
to look for other clusters of galaxies, besides the Norma cluster, in the zone of avoidance.
This was called the CIZA project for Clusters In the Zone of Avoidance. But all of the clusters
found where minor and didn’t resolve the mass problem. There was still not enough mass there.
As it turned out the overall situation was more complex. While the Norma cluster lies
near the center of the great attractor, there is another more significant piece to the puzzle. It
so happens that something is also attracting the great attractor along with our galaxy.
In fact this something is attracting the Great Attractor, our galaxy and all the galaxies in our
region of the universe. We are all moving toward a giant cluster of 8000 galaxies
called the Shapley Supercluster or concentration. This supercluster was discovered in the 1930s
by American astronomer Harlow Shapley. It is about 650 million light years away in
the Centaurus constellation, and has a total mass of more than ten million billion Suns,
or about 10 times the mass of the great attractor. it is the most massive structure within a billion
light-years from our Milky Way Galaxy. The density of galaxies here is greater
than that of the great attractor, and so even though it’s more than half a billion light
years away, it still pulls us towards it. The CIZA project found that the Shapley
concentration is the main source of our galaxy’s and the local group of galaxies’ movement.
But hold on, it isn’t the end of the story, because there is another piece to the puzzle.
In December of 2016, the team that originally discovered the Norma cluster,
discovered another final piece of the puzzle, the Vela supercluster. Its mass is estimated to be
1,000 times the mass of our Milky Way galaxy and is about 870 million light years away. This Vela
supercluster is responsible for the last 50 km/s of our galaxy’s velocity. This is less than 10%,
but it’s the final piece to explain the mystery of our total overall movement.
So, to recap the situation is as follows: Our galaxy and our local group of galaxies are all
being pulled towards the great attractor, which turns out to be the gravitational focal point
of the bigger Laniakea supercluster, which is composed from the Virgo supercluster,
which we live in, the Hydra-Centaurs-Supercluster, which contains the Norma cluster that is near the
center of great attractor, and other clusters. Now the great attractor itself, together with us,
is being gravitationally pulled towards the Shapley concentration
in the Shapely supercluster, and this is believed to be the main contributing factor
And finally we are further being slightly attracted to
the Vela supercluster which is even further away. So, what does this mean for the future our galaxy?
Will our galaxy eventually all be absorbed in the great attractor or the Shapley cluster?
The surprising answer is actually no, because of the expansion of the universe. And it’s not
just that our universe is expanding, but it’s actually expanding at an accelerating
rate. In other words, the expansion rate is increasing every second of every day.
This is what dark energy is. This means spacetime itself is expanding, and is happening everywhere in our universe.
So even though we are moving towards the great attractor, we will never reach it
because the expansion between it and our galaxy will increase faster than our velocity towards it.
The reason why is the universe expanding (at an accelerating rate) and the energy
behind it is a mystery that we haven’t solved yet.
This is why it’s called Dark Energy, because we don’t know what it is.
Dark energy makes up around 69% of the universe and dominates the overall behavior
of cosmological objects over very large distances. On a local level galaxies and stars still collide
because dark energy is not strong enough to separate them.
For example, we expect our galaxy will collide with the andromeda galaxy in about
5 billion years. But on larger scales, many things which are flying towards each other
will never actually reach each other because of the accelerating expansion.
Now this overall expansion is uniform. It’s the same in all directions.
Once you get to big enough scales, galaxies just move out without any preferred direction.
However, while most observations of the large-scale universe confirm this uniform
expansion, cosmologists have found some inconsistencies. It appears that
many galaxies might be moving in a preferred direction, instead of just uniformly outwards.
This is referred to as “Dark Flow” because it’s an unexpected motion
observed in the behavior of galaxies. It begs the question, what’s making them go in that direction?
There are some far out theories like there could be something frighteningly massive
just outside the edge of the visible universe, pulling at everything. Some speculate that it
could be caused by some catastrophe in the early universe, setting up a motion
that took billions of years to develop. Or, probably more it could just be a
mistake in the data, since our observations at such far distances are not perfect.
Right now evidence seems to indicate that it may not be a real thing,
but just objects moving around due to the local effects of gravity.
But let me leave you by answering the question I asked at the beginning, what does this mean for
earth and us humans? The answer is not much. Any movement due to Dark flow and the Great
Attractor are nothing to worry about. These movements so slow and the effect so tiny that it’s
not going to make any difference to us or even our solar system. There is no existential threat.
The universe is not about to tear apart. It’s just a former mystery that’s interesting to think about
and solve, but you can still sleep soundly at night. And if you have any questions or comments,
please leave them in the comments section. I’ll see you in the next video my friend.