Why do measurements of the universe's current expansion rate (the Hubble constant) using the early universe (cosmic microwave background plus the standard cosmological model) consistently give a lower value (~67-68 km/s/Mpc) than measurements using nearby stars and supernovae (~73 km/s/Mpc)?
#astrophysics
Post a problemDo ripples in spacetime predicted by Einstein's general relativity — gravitational waves produced by cataclysmic events like merging black holes — actually exist and can they be directly measured on Earth?
Why did NASA's Pioneer 10 and 11 spacecraft, as they left the solar system in the 1980s-90s, experience a tiny unexplained extra deceleration (about 8.7 x 10^-10 m/s^2) toward the Sun that didn't match standard gravitational predictions?
Why did several NASA and ESA spacecraft (including Galileo, NEAR, Cassini, and Rosetta) show small unexplained changes in speed (a few mm/s) during close Earth gravity-assist flybys in the 1990s-2000s, beyond what standard orbital mechanics predicts?
What is dark matter actually made of — is it a new fundamental particle (like a WIMP or axion) that barely interacts with ordinary matter, or does the evidence for it instead point to gravity itself working differently than Einstein and Newton predicted on galactic scales?
What is causing the expansion of the universe to accelerate — is it a constant vacuum energy density (the cosmological constant), a dynamical field (quintessence) that changes over time, or a sign that general relativity itself needs modification on cosmic scales?
Why does the observable universe consist almost entirely of matter with essentially no primordial antimatter, when the Big Bang should have produced matter and antimatter in equal amounts that would have mutually annihilated?
Does Einstein's general theory of relativity correctly describe gravity in every regime tested so far, including the strongest fields near black holes and at cosmological scales?
Can ripples in spacetime itself, predicted by general relativity and generated by events like merging black holes, be directly measured on Earth?
Do black holes really exist exactly as general relativity predicts, with event horizons as described, and do hypothetical thread-like 'cosmic strings' left over from the early universe exist?
What is the invisible 'dark matter' that makes up most of the gravitating mass in galaxies and the universe, and can it be directly detected in a laboratory?
What cataclysmic events power gamma-ray bursts, the most luminous explosions in the universe, and are long and short bursts produced by different mechanisms?
Origin mechanism of fast radio bursts
Research level
What physical process produces fast radio bursts — millisecond-long, extremely bright pulses of radio waves arriving from distant galaxies — and why do some sources repeat unpredictably while the majority appear to burst only once?
Black hole information paradox
Research level
When a black hole evaporates via Hawking radiation, does the information about what fell into it get destroyed forever (violating quantum mechanics), or is it somehow preserved and encoded in the outgoing radiation?
Missing baryon problem
Research level
Where is the roughly 30-40% of the universe's ordinary (baryonic) matter that Big Bang nucleosynthesis and cosmic microwave background measurements predict should exist, but which could not be directly observed in stars, galaxies, and cold gas in the more recent universe?
The cosmological constant, dark energy, and inflation
Research level
What is causing the expansion of the universe to accelerate (dark energy, or a cosmological constant), and did the universe undergo a brief phase of exponential inflationary expansion in its first fraction of a second?
Neutron star interiors and supernova explosion mechanisms
Research level
What is the exact state of matter inside a neutron star (its equation of state), and what physical mechanism actually revives a massive star's collapsing core into a supernova explosion?
Quasars, supermassive black holes, and galaxy formation
Research level
How do the supermassive black holes that power quasars and sit at galaxy centers form and grow, and how do galaxies themselves assemble over cosmic time?
What happens when two satellites collide head-on in orbit?
Research level
If two artificial satellites moving in opposite directions along intersecting orbits collide directly, what happens to the debris, and does this pose any longer-term hazard to other satellites?
If the Sun suddenly vanished, would Earth feel the loss of its gravitational pull at the same instant the light disappeared, or would there be a delay -- and can this be measured?