CAN THE GREAT ATTRACTOR DEFEAT DARK ENERGY?
The universe is beautiful and massive at the same time. The universe began 13.8
years ago when it was a tiny dense fireball that exploded. The Big Bang theory which is
our best and widely accepted theory of how the universe began, states that, everything we know
of was an infinitesimally small singularity, a point of infinite denseness and infinite heat.
Today the universe is still evolving. We humans have been around only for a very small fraction
of the timescale of the universe. Still we have observed a lot of the things and are continuously
learning about the mysteries of the universe. The hubble deep field is the best example of how much
we are able to see and that there are greater forces acting in our universe. Two such forces
are Gravity and dark energy; both very crucial for the destiny of our universe. In this video we will
talk about a strange place in the universe that is attracting everything: The great attractor!!
On the largest scales of the universe, the universe is expanding at an accelerating rate.
This line itself means a lot for our future. Scientists in the 23th century assumed the
expansion of the universe had to slow down, as gravity of all matter in the universe will
pull it back onto itself. But when Hubble turned his telescope towards the sky he was baffled. He
observed that the farther a galaxy was the faster it was moving away from us. This conclusion took
everyone by surprise. What could be responsible for such a phenomenon? Scientists eventually found
out that an invisible force that makes up 70% of the entire universe and is responsible for
driving distant galaxies farther and farther apart over time and causing them to speed up relative
to one another's perspective. However, gravitation causes matter to clump together, as our own galaxy
and local group do, and can resist this expansion if a large enough mass of matter clumps together
in one spot. However, galaxies and groupings are not the largest structures we are aware of.
There are also clusters and superclusters of galaxies in the Universe, and some of
them are right in our own backyard! Will one of them be able to fight evil energy in the end?
Thousands and thousands of galaxies are constantly tugging on us. Will they eventually draw us in,
despite the dark energy? Or will dark energy drive us to expand quickly enough to prevent
that from happening? To address this question, we must consider three factors: the expansion
of the Universe, local flaws in that motion, and what the Universe appears like near us.
THE EXPANSION OF THE UNIVERSE- As mentioned earlier, back in 1920
Hubble’s observations baffled every scientist. Hubble gave Hubble's law which defines how
fast any galaxy is receding away from us. In the spiral-shaped objects observed in the sky, Edwin
Hubble was able to identify a known class of star — Cepheid variables. They looked to brighten and
fade periodically over time, with a certain length of time inherent in each star. These stars all
have a brightness/time period relationship, which means that if you can measure that time period and
their apparent brightness, you can figure out how far away each star, and hence the galaxy it's in,
is from you. This is known as a standard candle, and we've advanced from Cepheids to other galaxies
to type Ia supernovae as the brightest, most easily identified standard candles out there.
Through these methods, we've discovered a relationship known as Hubble's law in all
directions we look: the speed at which an object appears to move away from us is proportional to
the Hubble parameter multiplied by the distance to that object. You may have heard it referred to
as the "Hubble constant," and it was a good way to think about it in the years and decades before the
Hubble Space Telescope, because we had only seen roughly halfway across the Universe at the time.
But the more we examined, the more we saw that the Universe's expansion was not only changing through
time, but it was also speeding up, indicating that there was more to the Universe than matter,
radiation, and curved space alone. Instead, the Universe today is made up of roughly 70%
dark energy, which is becoming increasingly vital as time passes.
Dark energy was not visible half a billion years ago because it made up such a small percentage
of total energy density. However, when matter and radiation dilute and lose density,
dark energy takes over as the Universe expands, generating the acceleration we see today. It
means that any structures that aren't already gravitationally connected – that haven't become
denser-than-average by a significant amount – will never be bonded together in our Universe. Instead,
they would accelerate away in accordance with the expansion of the Universe.
LOCAL IMPERFECTIONS IN THE MOTION OF THE UNIVERSE-
Even when we talk about the universe on larger scales of millions of light years,
gravitation had plenty of time to play a role in shaping up the universe. Trillions
of star clusters and hundreds of billions of galaxies emerged in the first few billion
years after the Big Bang, as the Universe's large-scale structure became rich and complex.
The most dense regions grew not just into galaxies, but also into groups and clusters
of tens, hundreds, or thousands of galaxies, all joined together in one massive region.
The gravitational pull of these flaws is extremely important. When we look out at a galaxy,
such as Andromeda, our nearest neighbour, we see it from a distance of 2.5 million light years.
It should be travelling away from us based on the expansion of the Universe.
However, if these two galaxies are huge enough, the gravitational attraction of the Milky Way on
Andromeda – and of Andromeda back on us in the Milky Way – can counteract the expansion. We'll
become gravitationally bonded together if the attraction force between them is large enough,
and it was large enough early on. Although dark energy may push other galaxies away from us,
we will eventually collide and fuse into a single massive structure throughout
time. This is gonna happen!! This is the reality of the fate of our Local group.
To get to Bob's point, the main question is what's going on with the great attractor and the nearest
clusters and superclusters to our location. To do so, we must first map out the neighbouring,
local Universe. But one thing is sure that on smaller scales of galaxies that are nearby,
gravitational force dominates over dark energy. As the Hubble law itself says, the farther away
a galaxy is the faster it is receding away from, as on larger distances dark energy dominates.
HOW THE UNIVERSE LOOKS LIKE NEAR US- ( We have talked about the universe on a
larger scale and how it is expanding. We also have talked about the local imperfections. Let's take a
look at how the universe looks like near us. We've accomplished this with greater than 80% accuracy!
(The galaxies positioned behind the galactic plane, which are difficult to observe from our
vantage point, are the sections we've missed.) We can look at three things at the same time:
All of the individual galaxies in our vicinity, and their movements relative to us.
Hubble's expansion of the Universe, when combined with galactic distances,
can be used to calculate how far these galactic motions deviate from Hubble's law.
The measured and inferred masses of everything we see around us, as well as the masses that
must be present at what locations in the Universe to generate the motions we perceive.
So we map out the local Universe in terms of position and velocity, as well as the local mass,
and we observe how and why things move. This helps us map out the local universe. The cosmic flows
project recently compiled all of this data and determined that the Milky Way is bound as part
of the local group, that our group is one of many nearby but outside of the Virgo Cluster,
and that all of these groups and clusters, along with a few others, form the Laniakea supercluster.
The mass must exist in order to explain the motions of these local structures,
where the missing mass was previously referred to simply as the "Great Attractor"
because the motions we observed did not correspond to the masses we discovered.
The very big structure – the collection of galaxies in Laniakea responsible for
this tremendous, attractive pull – forces the local group and many other galaxies in our local
supercluster to gravitate toward this mass. They deviate from the Hubble flow by many hundreds of
kilometres per second. It's a genuine force with a significant influence, and it acts to combat the
Hubble expansion and dark energy. But it fails.
The attractive pull of dark energy and the current expansion of the Universe is not
simply stronger than the attractive pull of the local supercluster; it is a no-brainer. The
unusual velocity, or deviation from the Hubble expansion, is only around 20% of what it would
need to be to link us to this massive object. In truth, the structure isn't really constrained;
this supercluster is simply an apparent structure, and Laniakea will dissolve as the Universe grows.
So the answer to the Big question is that we are being drawn towards Laniakea,
towards the "Great Attractor," but the force with which we are being drawn is
pathetically insufficient to cause us to fall in. All it can do is force the supercluster
to move away from us at a slightly slower rate than usual, staying within our reach for a few
billion years longer than an equidistant galaxy on the opposite side of the sky.
Laniakea is genuine and vast, but it is also transient, and it isn't massive enough to hold
itself together or finally draw us in. After all, our small group's fate is a lonely one. But what
do you think about the fate of our local group? How important is dark energy in our universe?
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