The Local Group is our pocket of the Universe, the limit we will never cross. But if we took the DNA of all the humans present on Earth in 2021, untangled and put these strands together into one long ribbon, it would span one and a half times across the Local Group. Bunched up tightly together, our species’ collective DNA could fill a cube with sides the length of a football field.
Clusters are families of entire galaxies, that unlike real families aren’t bound together by blood, petty feuds and love, but gravity. They contain enough matter to pull even distant family members toward them — again, with gravity, not a bad conscience and wedding ceremonies like real families do. Hanging out 60 million light years away, you could call this cluster of galaxies the next neighborhood over from ours.
If the observable Universe is our world, the Local Group of galaxies our neighborhood, and the Milky Way our house, the Virgo Supercluster is our city. The “city” has more than 700 galaxies in 100 Local Group “neighborhoods”, and stretches 110 million light years across. To put that in perspective, if the observable Universe is a sphere with a diameter of 1 km, large enough to contain the tallest skyscrapers, the Virgo Supercluster is a sphere 1.47 m across, about the height of a child. On that scale, the Milky Way is a tiny disk 2 mm across.
Space looks calm, but actually our own galaxy, the Milky Way, is traveling through the cosmos at 2.2 million km an hour. This is a bit of a mystery. According to our ideas about the Big Bang, everything in the Universe should be flying apart at similar speeds. If everything is moving at similar speeds, it should look like nothing is really moving at all. When some objects seem to be moving faster than others, it’s usually because clumps of matter like galaxy clusters are exerting an additional gravitational pull. But we know of nothing close to the Milky Way that explains its drift — we just know that something must be causing it. Astronomers named the mysterious clump of matter we haven’t found yet “The Great Attractor”.
“The Great Nothing” is an enormous sphere in the Universe famous for containing a very sparse number of galaxies — space’s version of the Australian Outback.
No one knows for sure why the Great Nothing exists. The most fun (totally unfounded) hypothesis is that perhaps the void is created by an expanding Kardashev III scale civilization that has been around long enough to conquer a major region of the Universe and capture the energy of its galaxies.
If the observable Universe is our world, the Local Group of galaxies our neighborhood, the Virgo Supercluster our city, and the Milky Way our house, the Pisces Cetus Supercluster Complex is our country. The PCSC is a “galaxy filament” — a cluster of galaxy superclusters — and the largest known structure in the Universe. The PCSC, our home filament, is one billion light years across, and it’s thought to contain mass equivalent to 10^18 Suns. Our “city”, the Virgo Supercluster, only comprises 0.1% of the PCSC’s total mass.
If the observable Universe is our world, the Local Group of galaxies our neighborhood, the Virgo Supercluster our city, and the Milky Way our house, the Pisces Cetus Supercluster Complex is our country. The PCSC is a “galaxy filament” — a cluster of galaxy superclusters — and the largest known structure in the Universe. The PCSC, our home filament, is one billion light years across, and it’s thought to contain mass equivalent to 10^18 Suns. Our “city”, the Virgo Supercluster, only comprises 0.1% of the PCSC’s total mass.
A parsec is a length unit used to measure the huge distances of astronomical objects outside the Solar System. A billion parsecs, or about 3.3 billion light years, are called a gigaparsec. Until we discover a multiverse, this is the largest unit of length measurement we’ll ever need. A gigaparsec makes the observable Universe seem quaint, as it stretches only about 28 gigaparsecs across, or about 14,000 cubic gigaparsecs in volume.
Galaxy filaments are clusters of galaxy superclusters, which makes them the largest known structures in the Universe. The appropriately—named Hercules Corona Borealis Great Wall is the largest known galaxy filament — making it the largest known structure in existence.
Galaxy filaments are kind of like the Universe’s countries. If our home country, the Pisces Cetus Supercluster Complex, is the size of Germany, the Hercules Corona Borealis Great Wall is around the size of Russia. An outrageous 10 billion light years across, the HCB Great Wall stretches over a tenth of the way across the observable Universe!
In 1995, scientists picked out a tiny section of the night sky — the amount that would be covered by a tennis ball 100 m above you — that was unusually devoid of stars. To the naked eye, and even in a normal telescope, this region looked empty and black.
The scientists used the Hubble Telescope to take a 10—day long exposure of the empty region to find out what was out there deep in the blackness. They came back with an astonishing photo of over 10,000 galaxies, each one perhaps containing 100 billion or more stars. All in a pinpoint little square of the night sky.
Scientists used the info from this photo to postulate that the observable Universe contains over 100 billion galaxies. Today, that galaxy estimate has risen by 20 fold to 2 trillion, and it may continue to rise as we learn more. That suggests the total stars in the observable Universe to be somewhere between 10^33 and 10^25, or around 1 septillion stars.
To put that in perspective, people at the University of Hawaii spent an unreasonable amount of time calculating an estimate for the number of grains of sand on every beach in the world — 7.5 x 10^18 or 7.5 quintillion. That means that for every grain of sand on every beach on Earth, there are about 100,000 stars in the observable Universe.
Keishinrei