Sign up for the Starts With a Bang newsletter
Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all.
Over the last 100 years, we’ve discovered where our Universe came from.
A period of cosmic inflation stretched space flat, seeding the Universe with quantum fluctuations.
From a region of space as small as can be imagined (all the way down to the Planck scale), cosmological inflation causes space to expand exponentially: relentlessly doubling and doubling again with each tiny fraction-of-a-second that elapses. Although this empties the Universe and stretches it flat, it also contains quantum fluctuations superimposed atop it: fluctuations that will later provide the seeds for cosmic structure within our own Universe. What happened before the final ~10^-32 seconds of inflation, including the question of whether inflation arose from a singular state before it, not only isn’t known, but may be fundamentally unknowable.
Then inflation ended, converting its field energy into matter, antimatter, and radiation.
The quantum fluctuations inherent to space, stretched across the Universe during cosmic inflation, gave rise to the density fluctuations imprinted in the cosmic microwave background, which in turn gave rise to the stars, galaxies, and other large-scale structures in the Universe today. This is the best picture we have of how the entire Universe behaves, where inflation precedes and sets up the Big Bang. Unfortunately, we can only access the information contained inside our cosmic horizon, which is all part of the same fraction of one region where inflation ended some 13.8 billion years ago.
Afterwards, the Universe expanded and cooled, forming atoms, stars, galaxies, and humans.
Our Universe, from the hot Big Bang until the present day, underwent a huge amount of growth and evolution, and continues to do so. Our entire observable Universe was approximately the size of a modest boulder some 13.8 billion years ago, but has expanded to be ~46 billion light-years in radius today. The complex structure that has arisen must have grown from seed imperfections of at least ~0.003% of the average density early on, and has gone through phases where atomic nuclei, neutral atoms, and stars first formed, eventually giving rise to our Solar System, planet, life, and humans.
However, despite all we’ve learned, these five major mysteries remain.
A galaxy cluster can have its mass reconstructed from the gravitational lensing data available. Most of the mass is found not inside the individual galaxies, shown as peaks here, but from the intergalactic medium within the cluster, where dark matter appears to reside. More granular simulations and observations can reveal dark matter substructure as well, with the data strongly agreeing with cold dark matter’s predictions. Dark matter must be something distinct from either matter or antimatter; it cannot be a particle within the Standard Model. Without the gravitational effects of dark matter, most galaxies would tear themselves apart during episodes of major star-formation.
1.) How did matter win out over antimatter?
The early Universe was full of matter and radiation, and was so hot and dense that it prevented all composite particles, like protons and neutrons from stably forming for the first fraction-of-a-second. There was only a quark-gluon plasma, as well as other particles (such as charged leptons, neutrinos, and other bosons) zipping around at nearly the speed of light. This primordial soup consisted of particles, antiparticles, and radiation: a highly symmetric state. Today’s Universe, by comparison, is more asymmetric, with more matter than antimatter. Presently known physics does not account for this.
All known particle physics reactions only create or destroy matter and antimatter in equal amounts.
The difference between matter and antimatter is accounted for by charge conjugation symmetry: a discrete symmetry that exchanges particles for antiparticles and vice versa. Where this symmetry holds, there is an associated conserved quantity as a consequences of Noether’s theorem. Where that symmetry is violated, the conservation law no longer necessarily holds.
Today’s matter-dominated Universe remains an unsolved puzzle.
If we allow X and Y particles, high-energy bosons predicted to exist within the context of grand unified theories, to decay into the quarks and lepton combinations shown, their antiparticle counterparts will decay into the respective antiparticle combinations. If CP is violated, which it’s expected to be, then the decay pathways — or the percentage of particles decaying one way versus another — can be different for the X and Y particles compared to the anti-X and anti-Y particles, resulting in a net production of baryons over antibaryons and leptons over antileptons.
2.) What is dark matter, and how did it arise?
A galaxy that was governed by normal matter alone (left) would display much lower rotational speeds in the outskirts than toward the center, similar to how planets in the Solar System move. However, observations indicate that rotational speeds are largely independent of radius (right) from the galactic center, leading to the inference that a large amount of invisible, or dark, matter must be present. These types of observations were revolutionary in helping astronomers understand the necessity for dark matter in the Universe, and also explain the shapes and behavior of matter located within a galaxy’s spiral arms.
The cosmic structures we observe require abundant quantities of dark matter.
The X-ray (pink) and overall matter (blue) maps of various colliding galaxy clusters show a clear separation between normal matter and gravitational effects, some of the strongest evidence for dark matter. The X-rays come in two varieties, soft (lower-energy) and hard (higher-energy), where galaxy collisions can create temperatures ranging from several hundreds of thousands of degrees up to ~100 million K. Meanwhile, the fact that the gravitational effects (in blue) are displaced from the location of the mass from the normal matter (pink) shows that dark matter must be present. Without dark matter, these observations (along with many others) cannot be sufficiently explained.
Yet dark matter’s nature, and its origins, remain unexplained.
The cosmic web that we see, the largest-scale structure in the entire Universe, is dominated by dark matter. Simulations of the large-scale structure of the Universe must include both dark matter and normal matter, including the effects of star-formation, feedback, and gas infall, as all of them are needed in order to predict the emergence of visible structures. Identifying which regions are dense and massive enough to correspond to star clusters, galaxies, galaxy clusters, and filaments, as well as determining when and under which conditions they form, is one of the great achievements of modern cosmology.
3.) When did the first stars form, and what were they like?
This graph shows the combination of the Hubble, JWST NIRCam and JWST NIRSpec data for galaxy RXJ2129-z8HeII. There is an unusually strong, blue tilt to the stellar spectrum of this object, but the evidence for any pristine material amidst the highly enriched gas and stars that are present is too flimsy to make a compelling case for the presence of any pristine, Population III (a.k.a., the “first”) stars. No such population, as of 2025, has yet been found.
Despite sensational claims, no truly pristine stars have been observed.
An artist’s conception of what a region within the Universe might look like as it forms stars for the first time. As stars shine, accumulate matter, and contract, radiation will be emitted, both electromagnetic and gravitational. Inside the star, gas pressure fights against gravitation, holding the various interior layers up against gravitational collapse. Surrounding the star-forming region is darkness, as neutral atoms effectively absorb that emitted starlight, while the emitted ultraviolet starlight works to ionize that matter from the inside out.
We’ve yet to discover when they first ignited, or what properties they possessed.
This illustration shows an example of one of the first stars in the Universe, turning on and shining brilliantly while surrounded by a cocoon of neutral gas. Without metals to cool them down or radiate energy away, only large-mass clumps in the heaviest-mass regions can form stars. The very first stars of all likely formed when the Universe was just 30-to-100 million years of age, or between a redshift of around 30-to-70. For comparison, JWST has only seen back to 285 million years after the Big Bang, or a redshift of 14.
4.) What type of inflation occurred?
According to the most sensitive constraints we have, from the latest BICEP/Keck data, the red shaded area is all that’s permitted as far as inflationary models go. Theorists have been mucking around in regions that can soon be excluded (green, blue), but viable values of r can be as small as we care to build our models. The green curve, as well, can be extended farther down in many models. The tighter our constraints in this parameter space, the better we can rule out various classes of inflationary models.
Many inflationary models are compatible with our observations.
This map shows the CMB’s polarization signal, as measured by the Planck satellite in 2015. The top and bottom insets show the difference between filtering the data on particular angular scales of 5 degrees and 1/3 of a degree, respectively. While temperature data, alone, can demonstrate that the CMB is of cosmic nature, the polarization signal gives us key pieces of information relevant to the details of cosmic inflation, including which “flavors” of inflation are allowed and disallowed.
Future CMB and lensing observatories could further constrain inflation’s nature.
This image shows the Large Aperture Telescope’s colossal, 6-meter primary and secondary mirrors at the Simons Observatory in February of 2025. The telescope has already seen first light, and will soon begin delivering new CMB science as never before.
5.) What happened before inflation?
From whatever pre-existing state started it, inflation predicts that a series of independent universes will be spawned as inflation continues, with each one being completely disconnected from every other one, separated by more inflating space. One of these “bubbles,” where inflation ended, gave birth to our Universe some 13.8 billion years ago. Today, dark energy dominates the Universe and causes space to expand exponentially as well. These scenarios may be related, but we have no idea how long inflation persisted for prior to the hot Big Bang: only the ability to say, “at least 10^-32 seconds” or so.
The inflationary state itself is necessarily past-timelike-incomplete
If you extrapolate a universe filled with radiation (blue), matter (red), or inflationary field energy (yellow) backwards in time, you’ll find that the matter or radiation filled states terminate in a singularity at some finite time in the past. The inflationary state, however, never does so. It was proven in 2001 that all inflationary spacetimes are past-timeline-incomplete, mandating that some other state predate the onset of cosmic inflation.
How it arose, and whether spacetime had an origin event, may never be known.
In the earliest known stages of cosmic history, a period of relentless exponential expansion known as cosmic inflation occurred, where inflation’s end corresponds with our initial hot Big Bang. The inflationary state lasted at least approximately ~10^-32 seconds, but could have lasted for far longer. Some other state, however, must have predated the inflationary one, and science has not, and possibly cannot, determine whether that state was singular or non-singular in origin.
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.
Sign up for the Starts With a Bang newsletter
Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all.
