Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all
Which is good, because if they do, they violate the cosmological principle.
In theory, the Universe should be the same, on average, everywhere.
A simulation of the large-scale structure of the Universe. While, on small scales, various regions are dense and massive enough to correspond to star clusters, galaxies, and galaxy clusters, while others correspond to cosmic voids, on larger scales, every location is largely similar to every other location. (DR. ZARIJA LUKIC)
On the largest scales, it shouldn’t matter which direction you observe.
This image shows a map of the full sky and the X-ray clusters identified to measure the expansion of the Universe in a direction-dependent way, along with four X-ray clusters in detail imaged by NASA’s Chandra X-ray observatory. Although the results suggest the Universe’s expansion may not be isotropic, or the same in all directions, the data is far from clear-cut, and the anisotropic interpretation was heavily criticized. (NASA/CXC/UNIV. OF BONN/K. MIGKAS ET AL.)
Nor should it matter which location you’re examining.
In modern cosmology, a large-scale web of dark matter and normal matter permeates the Universe. On the scales of individual galaxies and smaller, the structures formed by matter are highly non-linear, with densities that depart from the average density by enormous amounts. On very large scales, however, the density of any region of space is very close to the average density: to about 99.99% accuracy. (WESTERN WASHINGTON UNIVERSITY)
We expect isotropy and homogeneity, with physical consequences if they’re violated.
The early Universe was full of matter and radiation, and was so hot and dense that the quarks and gluons present didn’t form into individual protons and neutrons, but remained in a quark-gluon plasma. This primordial soup consisted of particles, antiparticles, and radiation, and although was in a lower entropy state than our modern Universe, there was still plenty of entropy. (RHIC COLLABORATION, BROOKHAVEN)
Initially, the Big Bang simultaneously occurred everywhere.
The full suite of what’s present today in the Universe owes its origins to the hot Big Bang. More fundamentally, the Universe we have today can only come about because of the properties of spacetime and the laws of physics. Without them, we cannot have existence in any form. (NASA / GSFC)
All locations possessed equivalent temperatures and densities.
As our satellites have improved in their capabilities, they’ve probes smaller scales, more frequency bands, and smaller temperature differences in the cosmic microwave background. The temperature imperfections help teach us what the Universe is made of and how it evolved, painting a picture that requires dark matter to make sense. (NASA/ESA AND THE COBE, WMAP AND PLANCK TEAMS; PLANCK 2018 RESULTS. VI. COSMOLOGICAL PARAMETERS; PLANCK COLLABORATION (2018))
Only tiny, 1-part-in-30,000 imperfections get superimposed atop them.
The large-scale structure of the Universe changes over time, as tiny imperfections grow to form the first stars and galaxies, then merge together to form the large, modern galaxies we see today. Looking to great distances reveals a younger Universe, similar to how our local region was in the past. The temperature fluctuations in the CMB, as well as the clustering properties of galaxies throughout time, provide a unique method of measuring the Universe’s expansion history. (CHRIS BLAKE AND SAM MOORFIELD)
Those imperfections then evolved gravitationally, limited by our physical laws.
This snippet from a structure-formation simulation, with the expansion of the Universe scaled out, represents billions of years of gravitational growth in a dark matter-rich Universe. Note that filaments and rich clusters, which form at the intersection of filaments, arise primarily due to dark matter; normal matter plays only a minor role. (RALF KÄHLER AND TOM ABEL (KIPAC)/OLIVER HAHN)
Tremendous cosmological structures formed: stars, galaxies, and the great cosmic web.
A map of more than one million galaxies in the Universe, where each dot is its own galaxy. On these large scales, it becomes clear that the clustering patterns we see are important on small cosmic scales, but as we look to larger and larger scales, the Universe appears more uniform. (DANIEL EISENSTEIN AND THE SDSS-III COLLABORATION)
We expect a structural size limit: ~1.2 billion light-years.
The 3D reconstruction of 120,000 galaxies and their clustering properties, inferred from their redshift and large-scale structure formation. The left, black-and-white image is the raw data, the green dots show the reconstructed 3D positions of those same galaxies. (JEREMY TINKER AND THE SDSS-III COLLABORATION)
Anything larger wouldn’t have sufficient time to form.
Both simulations (red) and galaxy surveys (blue/purple) display the same large-scale clustering patterns as one another, even when you look at the mathematical details. If dark matter weren’t present, a lot of this structure would not only differ in detail, but would be washed out of existence; galaxies would be rare and filled with almost exclusively light elements. (GERARD LEMSON AND THE VIRGO CONSORTIUM)
The warm-hot intergalactic medium (WHIM) has been seen along incredibly overdense regions, like the Sculptor wall, illustrated above. These walls are enormous, but no larger than 1.4 billion light-years, at least as have been confirmed to exist. Still, it’s conceivable that there are still surprises out there in the Universe. (SPECTRUM: NASA/CXC/UNIV. OF CALIFORNIA IRVINE/T. FANG. ILLUSTRATION: CXC/M. WEISS)
A region of space devoid of matter in our galaxy reveals the Universe beyond, where every point is a distant galaxy. The cluster/void structure can be seen very clearly, demonstrating that our Universe is not of exactly uniform density on all scales. Everywhere we look, however, we still find ‘something’ in the Universe. (ESA/HERSCHEL/SPIRE/HERMES)
This figure shows the relative attractive and repulsive effects of overdense and underdense regions on the Milky Way. Note that, despite the large number of galaxies clumped and clustered nearby, there are also large regions that have extremely few galaxies: cosmic voids. While we have a few substantial ones nearby, there are even larger and lower-density voids found in the distant Universe, but nothing defying our cosmic expectations. (YEHUDA HOFFMAN, DANIEL POMARÈDE, R. BRENT TULLY, AND HÉLÈNE COURTOIS, NATURE ASTRONOMY 1, 0036 (2017))
But two classes of structures threaten this picture.
Some quasar groupings appear to be clustered and/or aligned on larger cosmic scales than are predicted. The largest of them, known as the Huge Large Quasar Group (Huge-LQG), consists of 73 quasars spanning up to 5–6 billion light-years, but may only be what’s known as a pseudo-structure. (ESO/M. KORNMESSER)
Here, two different large quasar groupings are shown: the Clowes-Campusano LQG in red and the Huge-LQG in black. Just two degrees away, another LQG has been found as well. however, whether these are just unrelated quasar locations or a true larger-than-expected set of structures remains unresolved. (R. G. CLOWES/UNIVERSITY OF CENTRAL LANCASHIRE; SDSS)
NASA’s Fermi Satellite has constructed the highest resolution, high-energy map of the Universe ever created. Without space-based observatories such as this one, we could never learn all that we have about the Universe, nor could we even accurately measure the gamma-ray sky. Some gamma-ray bursts appear to be clustered in a way that may indicate larger-than-expected cosmic structures. (NASA/DOE/FERMI LAT COLLABORATION)
If real, these structures defy our present cosmic understanding.
This illustration of the large GRB ring, and the inferred underlying large-scale structure, shows what might be responsible for the pattern we’ve observed. However, this may not be a true structure, but only a pseudo-structure, and we may be fooling ourselves by believing this extends across many billions of light-years of space. (PABLO CARLOS BUDASSI/WIKIMEDIA.ORG)
However, they may be purely phantasmal.
This illustration of the most distant gamma-ray burst ever detected, GRB 090423, is thought to be typical of most fast gamma-ray bursts. However, whether the multiple gamma-ray bursts we’ve seen are good tracers of the underlying large-scale structure or not remains a debated topic. (ESO/A. ROQUETTE)
Combination image of quasar RX J1131 (center) taken via NASA’s Chandra X-ray Observatory and the Hubble Space Telescope. Microlensing events associated with this quasar provide evidence for some ~2,000 rogue/orphan planets populating the interstellar space around this quasar’s core, making this the most distant location known that contains planets. While other quasars and structures can be found nearby, we can tell that this object isn’t part of a structure that’s larger than the expected cosmic limits. (NASA/CXC/UNIV OF MICHIGAN/R.C.REIS ET AL)
Only superior data, sufficiently mapping out our Universe, will decide.
The Hubble Ultra-Deep Field, shown in blue, is currently the largest, deepest long-exposure campaign undertaken by humanity. For the same amount of observing time, the Nancy Grace Roman Telescope will be able to image the orange area to the exact same depth, revealing over 100 times as many objects as are present in the comparable Hubble image. We should finally be able to test whether these quasar and gamma-ray burst clusterings are real structures, or just pseudo-structures. (NASA, ESA, AND A. KOEKEMOER (STSCI); ACKNOWLEDGEMENT: DIGITIZED SKY SURVEY)
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.
