Does the universe keep extending endlessly into the abyss of space, or does it have a defined end?
Of all the scientific questions you may ponder, "Is the universe infinite?" is one of the hardest. It is impossible to answer with certainty at this point. Scientists have proposed both possibilities, and each has its own supporters and detractors. Determining whether the universe has some kind of boundary ultimately depends on figuring out its shape, size, and how much of it we can actually observe.
Is the universe infinite? And what shape is it?
The shape of the universe would have a lot to do with its size. Cosmologists have theorized that a universe would likely come in one of three possible shapes, which are dependent on the curvature of space. As described in Discover Magazine, the universe could be flat, having no curvature, but spatially infinite. Or it could be open, shaped like a saddle (with negative curvature) and also infinite. Or it could be closed, look like a sphere, and be spatially finite.
So which shape really is it? Nobel Prize-winning cosmologist John Mather of NASA's Goddard Space Flight Center, also the chief scientist for the James Webb Space Telescope, maintains that recent observations of cosmic microwave background radiation (CMB) remaining from the time of the Big Bang support the idea of the universe being flat, without any curvature (at least to the limit of what is observable).
"The universe is flat like an [endless] sheet of paper," shared Mather. "According to this, you could continue infinitely far in any direction and the universe would be just the same, more or less."
The geometry of the universe is determined by the density parameter Ω within cosmological Friedmann Equations.Credit: NASA / WMAP Science Team
Measuring the size of the universe
Current calculations say that the observable universe extends 46.5 billion light-years in every direction, making its diameter 93 billion light-years across.
Consider this: The age of the universe is 13.8 billion years, which means it took 13.8 billion light-years for the light from the farthest edge of the observable universe to reach us. But in that time, the universe has continued to expand at a rate that appears to be speeding up. Now, the edge of the observable universe has moved and is 46.5 billion light-years away.
That vast amount of space includes anywhere from 200 billion to 2 trillion galaxies, according to varying estimates. And each galaxy has, on average, around a 100 billions stars.
These gargantuan numbers are almost impossible to grasp. How did scientists come up with them?
As shared in an interview with BBC by Caitlin Casey, an astronomer at the University of Texas at Austin, scientists use a variety of tools and methods called "the cosmic distance ladder" to estimate distances between objects in the vastness of space. They start out with distances they can actually measure directly, like through bouncing radio waves off nearby bodies in the solar system, noting the time required for the waves to come back to Earth.
For distances that are harder to gauge, like those for galaxies at the boundary of the universe, astronomers utilize inferences based on calculations and observational evidence.
For instance, they employ "parallax measurement" that relies on measuring a star's shift in relation to objects in its background, as well as "main sequence fitting," which takes advantage of our knowledge of stellar evolution. (Stars evolve over time, changing size and brightness.) Knowledge of how brightness is connected to distance is paramount in determining the location of distant objects. So is analysis of redshift, which involves measuring changes in the wavelengths of light coming from faraway galaxies.
What about the unobservable universe?
If you notice, the numbers above pertain to the observable universe, the ball-like part of the universe that can be somehow seen from Earth or detected using our space telescopes and probes. But what about parts of the universe we cannot see? Some portions of the universe may be just too far away for the light emitted after the Big Bang to have had sufficient time to reach us here on Earth.
One study from a group of UK scientists estimated that if you take that into account, the actual size of the universe could be at least 250 times larger. They found that if you refer to space in terms of a so-called Hubble volume, which is similar to the volume of space in the visible universe, a closed and finite universe would contain roughly 250 to 400 Hubble volumes.
Another possibility entertained by scientists like Nobel Prize-winning Roger Penrose is that the Big Bang was just one of the periods of cosmic regeneration that our universe has experienced. There could have been multiple Big Bangs, followed by Big Crunches, periods in which a universe would stop expanding and collapse upon itself.
If all we know about the universe is derived from how it expands after the latest Big Bang, the questions if the universe is infinite or what size it may be are almost moot. As is often the case, more study and confirmation of our theories is needed.
Is there an edge to the universe?
Whether we have a finite universe or an infinite universe like an ever-expanding bubble, does it still have an "edge"? Is there some place you can go and say, "Yep, this is the end of the universe"? The simple answer is likely no.
As explained to LiveScience by Robert McNees, an associate professor of physics at Loyola University Chicago, the universe is isotropic. That means it follows the so-called "cosmological principle" and has the same properties and follows the same laws of physics in all directions.
If that is so, then the universe is much like the surface of a balloon. Imagine being an ant walking along a balloon. You wouldn't know there's an edge to it if you kept walking forward. You'd likely come back to where you started eventually, but the journey around and around could keep going without end.
If someone were to blow more air into the balloon as you keep walking along it, you'd experience some parts of the balloon moving farther away from you. Still, you'd be no closer to finding the balloon's edge.
Much like the ants, we're unlikely to get to the end of the universe. But we may still be able to answer one day "is the universe infinite" or does it have an actual boundary?
A new map derived with the help of artificial intelligence reveals previously unknown "bridges" linking galaxies in the local universe. The bridges are in the form of filamentary structures. The scientists hope their map, published along with their paper in the Astrophysical Journal, can provide fresh insights into dark matter and the history of our universe.
While dark matter is an accepted notion, thought to make up 80 percent of all the matter in the universe, it has been hard to find. Scientists have, however, inferred much about the existence and behavior of dark matter by observing its gravitational influence on other space objects.
The universe has a dark matter skeleton
Cosmologists believe that dark matter serves as the filamentary skeleton of the cosmic web, which in turn, makes up the large-scale structure of the universe that partially controls the motion of galaxies and other cosmic systems.
While it's not proven possible yet to directly measure how dark matter is distributed in our local universe, the international team behind the research used AI to create a new map. The "local universe," which includes us, is an area about 1 billion light-years in radius where galaxies and related space objects are "essentially frozen in their present day configurations" and cosmic evolution effects are negligible, the astronomers explain.
"Ironically, it's easier to study the distribution of dark matter much further away because it reflects the very distant past, which is much less complex," said one of the study's authors, Donghui Jeong, associate professor of astronomy and astrophysics at Penn State. "Over time, as the large-scale structure of the universe has grown, the complexity of the universe has increased, so it is inherently harder to make measurements about dark matter locally."
A map of dark matter within the local universe. Smaller filamentary features (yellow) act as hidden bridges between galaxies. Dark matter's gravitational influence on galaxies is indicated by black dots. Prominent features of the universe are shown by red dots and X marks the Milky Way. CREDIT: Hong et. al., Astrophysical Journal.
Creating a better dark matter map
Cosmic web maps created previously relied on simulating the 13.8-billion-year evolution of the universe from early stages to present day. Such efforts required a tremendous amount of computation and did not yet produce accurate representations of the local universe, leading researchers to devise a novel approach. For the new map, they focused on utilizing machine learning to create a model based on the distribution and motion of galaxies. This allowed them to estimate how dark matter is distributed.
The AI was trained on simulated galaxies similar to the Milky Way using Illustris-TNG — an ongoing series of simulations that features galaxies, dark matter, gasses, and other matter.
Jeong explained that if you feed specific information into the model, it can fill out the gaps, relying on what it has already processed. The scientists further confirmed the mapping by applying it to real local galaxy data from the Cosmicflows-3 catalog of distance information about nearly 18 thousand galaxies.
Hidden bridges
The resulting map features major structures in our local universe like the "local sheet," which contains the Milky Way. Nearby galaxies and the "local void" — a nearby region of empty space — are also represented. What's more, the map allowed researchers to spot new structures. In particular, they hope to study in greater depth the small filamentary structures they discovered that appear to link galaxies. Jeong called them "hidden bridges."
Jeong believes these filaments can provide insight into the future of our galaxy. One particular question of note is whether the Milky Way would eventually collide with the Andromeda galaxy.
"Because dark matter dominates the dynamics of the universe, it basically determines our fate," shared Jeong. "So we can ask a computer to evolve the map for billions of years to see what will happen in the local universe. And we can evolve the model back in time to understand the history of our cosmic neighborhood."
Further studies that include galaxy data from new astronomical surveys will be needed to perfect the map's accuracy.
Scientists found similarities in the workings of two systems completely different in scale – the network of neuronal cells in the human brain and the cosmic web of galaxies.
Researchers studied the two systems from a variety of angles, looking at structure, morphology, memory capacity, and other properties. Their quantitative analysis revealed that very dissimilar physical processes can create structures sharing levels of complexity and organization, even if they are varied in size by 27 orders of magnitude.
The unusual study was itself carried out by Italian specialists in two very different fields – astrophysicist Franco Vazza from the University of Bologna and neurosurgeon Alberto Feletti from the University of Verona.
"The tantalizing degree of similarity that our analysis exposes seems to suggest that the self-organization of both complex systems is likely being shaped by similar principles of network dynamics, despite the radically different scales and processes at play," wrote the scientists in their new paper.
One of the most compelling insights of the study involved looking at the brain's neuronal network as a universe in itself. This network contains about 69 billion neurons. If you're keeping score, the observable universe has a web of at least 100 billion galaxies.
Another similarity is the defined nature of their networks–neurons and galaxies–that have nodes connected by filaments. By studying the average number of connections in each node and the clustering of connections in nodes, the researchers concluded that there were definite "agreement levels" in connectivity, suggesting the two networks grew as a result of similar physical principles, according to Feletti.
Section of the human brain (left) and a simulated section of the cosmos (right).
Credit: University of Bologna
There are also interesting comparisons when it comes to the composition of each structure. About 77 percent of the brain is water, while about 70 percent of the Universe is filled with dark energy. These are both passive materials that have indirect roles in their respective structures.
On the flip side of that, about 30 percent of the masses of each system is comprised of galaxies or neurons.
The scientists also found an uncanny similarity between matter density fluctuations in brains and the cosmic web.
"We calculated the spectral density of both systems. This is a technique often employed in cosmology for studying the spatial distribution of galaxies," Vazza said in a press release. "Our analysis showed that the distribution of the fluctuation within the cerebellum neuronal network on a scale from 1 micrometer to 0.1 millimeters follows the same progression of the distribution of matter in the cosmic web but, of course, on a larger scale that goes from 5 million to 500 million light-years."
Check out the new study "The Quantitative Comparison Between the Neuronal Network and the Cosmic Web", published in Frontiers in Physics.
Michio Kaku: Consciousness Can be Quantified | Big Think
"Believe it or not, sitting on our shoulders is the most complex object that Mother Nature has created in the known universe. You have to go at least 24 trillion miles to the nearest star to find a planet that may have life and may have intelligence. And yet our brain only consumes about 20-30 watts of power and yet it performs calculations better than any large supercomputer." - Michio Kaku
The Milky Way, the galaxy that contains our Solar System, is estimated to be about 13.6 billion years old. But what was there before? A loaded question that scientists are getting closer to answering. A team of astrophysicists reconstructed the cosmic ancestry of our galaxy. They figured out its family tree by using artificial intelligence to analyze globular clusters that orbit the Milky Way.
Globular clusters are collections of up to a million stars, almost as old as the Universe itself. Over 150 clusters of this kind are present in the Milky Way. Scientists believe that many of them were created in smaller galaxies that merged to form our galaxy. Astronomers treat them as "fossils" for reverse engineering the history of our home in space. The latest study allowed the research team to do exactly that.
The group led by Dr. Diederik Kruijssen from the University of Heidelberg (ZAH) and Dr. Joel Pfeffer from Liverpool John Moores University modeled the merger story of the Milky Way. They designed sophisticated computer simulations called E-MOSAICS to represent a complete model of the creation, evolution and demise of globular clusters. The researchers linked ages, the chemistry, and orbital motions of these clusters to the composition of the preceding galaxies that formed them, over 10 billion years ago. The analysis allowed the scientists to pinpoint how many stars the progenitor galaxies had as well as when their merger forming the Milky Way took place.
"The main challenge of connecting the properties of globular clusters to the merger history of their host galaxy has always been that galaxy assembly is an extremely messy process, during which the orbits of the globular clusters are completely reshuffled," Kruijssen pointed out.
Check out how E-MOSAICS simulations shows the formation of a galaxy like the Milky Way:
Once they trained their artificial neural network to investigate the galactic merger history, the researchers "tested the algorithm tens of thousands of times on the simulations and were amazed at how accurately it was able to reconstruct the merger histories of the simulated galaxies, using only their globular cluster populations," according to Kruijssen.
To dive deep into the prehistory of our galaxy, the researchers directed their AI to study global clusters that they suspected were formed in the progenitor galaxies. The orbital motion of the clusters informed their predictions. The AI was able to pinpoint the masses of the stars and the details of the mergers with great precision. It also discovered a collision 11 billion years ago between the Milky Way and a mysterious galaxy the scientists evocatively dubbed "Kraken."
Credit: D. Kruijssen / Heidelberg University
Galaxy merger tree of the Milky Way. The main progenitor of the Milky Way is shown by the trunk of the tree, with color representing its stellar mass. Black lines show the five identified satellites. Grey dotted lines demonstrate other mergers that the Milky Way likely underwent, but could not be connected to a particular progenitor. From left to right, the six images at the top list the identified progenitor galaxies: Sagittarius, Sequoia, Kraken, the Milky Way's Main progenitor, the progenitor of the Helmi streams, and Gaia-Enceladus-Sausage.
Kruijssen called this collision with Kraken "the most significant merger the Milky Way ever experienced." This event would have superseded the collision with the Gaia-Enceladus-Sausage galaxy of 9 billion years ago and was likely much more transformative, since our galaxy at that time was four times less massive.
Overall, the researchers think the Milky Way consumed about five galaxies of over 100 million stars, as well as 15 galaxies with 10 million stars or more. The scientists hope their findings will be used to locate debris from all of our galactic ancestors.
Check out their study "Kraken reveals itself – the merger history of the Milky Way reconstructed with the E-MOSAICS simulations" published in Monthly Notices of the Royal Astronomical Society.
Dark matter is believed to be important stuff, the glue that holds together the dust, gas, and stars that make up galaxies. It's the organizing force for the universe's large-scale structure, the shape you'd see if you were able to zoom way, way out, and it comprises most of a galaxy's mass.
We don't know precisely what dark matter is, since it doesn't emit or reflect light, or absorb it for that matter, rendering it invisible to our instruments. However, we can see what dark matter does, insofar as light from objects behind dark matter warps and is magnified as it makes its way toward us. That visual distortion is referred to as dark matter's "lensing" effect, as it's similar to what you might see passing a magnifying glass over an object.
Now a new study of images from the Hubble Space Telescope combined with spectra from the European Southern Observatory's Very Large Telescope (VLT) in Chile finds that there's either a lot more dark matter than computer models predict, or there's a major puzzle piece missing from what we thought we knew about dark matter's behavior.
A trio of intriguing galaxy clusters
The three galaxy clusters imaged for the study
The discrepancy has to do with images of three galaxy clusters captured by Hubble's Wide Field Camera 3 and Advanced Camera for Surveys as part of two Hubble projects: The Frontier Fields and the Cluster Lensing And Supernova survey with Hubble (CLASH) programs. The three clusters are called MACS J1206.2-0847, MACS J0416.1-2403, and Abell S1063.
Such imagery can be used for authenticating — or exposing flaws in —predictive computer models of dark matter's behavior, locations, and concentrations.
Lead author Massimo Meneghetti of the INAF-Observatory of Astrophysics and Space Science of Bologna, Italy, says that "galaxy clusters are ideal laboratories in which to study whether the numerical simulations of the Universe that are currently available reproduce well what we can infer from gravitational lensing."
Mapping dark matter
The assumption has been that the greater the lensing effect, the higher the concentration of dark matter.
As scientists analyzed the clusters' large-scale lensing — the massive arc and elongation visual effects produced by dark matter — they noticed areas of smaller-scale lensing within that larger distortion. The scientists interpret these as concentrations of dark matter within individual galaxies inside the clusters.
The researchers used spectrographic data from the VLT to determine the mass of these smaller lenses. Pietro Bergamini of the INAF-Observatory of Astrophysics and Space Science in Bologna, Italy explains, "The speed of the stars gave us an estimate of each individual galaxy's mass, including the amount of dark matter." The leader of the spectrographic aspect of the study was Piero Rosati of the Università degli Studi di Ferrara, Italy who recalls, "the data from Hubble and the VLT provided excellent synergy. We were able to associate the galaxies with each cluster and estimate their distances."
This work allowed the team to develop a thoroughly calibrated, high-resolution map of dark matter concentrations throughout the three clusters.
But the models say...
However, when the researchers compared their map to the concentrations of dark matter computer models predicted for galaxies bearing the same general characteristics, something was way off. Some small-scale areas of the map had 10 times the amount of lensing — and presumably 10 times the amount of dark matter — than the model predicted.
"The results of these analyses further demonstrate how observations and numerical simulations go hand in hand," notes one team member, Elena Rasia of the INAF-Astronomical Observatory of Trieste, Italy. Another, Stefano Borgani of the Università degli Studi di Trieste, Italy, adds that "with advanced cosmological simulations, we can match the quality of observations analyzed in our paper, permitting detailed comparisons like never before."
"We have done a lot of testing of the data in this study," Meneghetti says, "and we are sure that this mismatch indicates that some physical ingredient is missing either from the simulations or from our understanding of the nature of dark matter." Priyamvada Natarajan of Yale University in Connecticut agrees: "There's a feature of the real Universe that we are simply not capturing in our current theoretical models."
Given that any theory in science lasts only until a better one comes along, Natarajan views the discrepancy as an opportunity, saying, "this could signal a gap in our current understanding of the nature of dark matter and its properties, as these exquisite data have permitted us to probe the detailed distribution of dark matter on the smallest scales."
At this point, it's unclear exactly what the conflict signifies. Do these smaller areas have unexpectedly high concentrations of dark matter? Or can dark matter, under certain currently unknown conditions, produce a tenfold increase in lensing beyond what we've been expecting, breaking the assumption that more lensing means more dark matter?
Obviously, the scientific community has barely begun to understand this mystery.
