cosmology

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Brief Answers to Cosmic Questions (~2000)
2 months ago
- The universe has no edge; galaxies extend uniformly in all directions as far as we can detect.
- Galaxies are not arranged on a sphere; the Big Bang was not an explosion in space but an expansion of space itself everywhere.
- We cannot see the whole universe because light from distant galaxies has not had time to reach us due to the universe's evolution and expansion.
- The expansion of the universe refers to the stretching of space between galaxy clusters, not galaxies moving through space.
- Evidence for the Big Bang includes the recession of galaxies, cosmic microwave background, and abundances of hydrogen and helium.
- The Big Bang scenario describes the early universe but does not explain the origin of space, time, or energy.
How Fast Is the Universe Really Expanding?
2 months ago
- The universe's expansion is accelerating due to dark energy, a discovery that won the Nobel Prize in 2011.
- The Hubble tension arises from a significant discrepancy between local measurements of the expansion rate (using supernovae) and predictions from the cosmic microwave background based on the Lambda-CDM model.
- The Lambda-CDM standard model is highly successful but may have cracks, as multiple independent measurements consistently show a mismatch.
- Adam Riess continues to refine distance measurements using the Hubble and James Webb Space Telescopes, finding no errors in local observations.
- Future telescopes like the Nancy Grace Roman, Euclid, and Vera Rubin are expected to provide new data to resolve the Hubble tension.
- The scientific process emphasizes redundancy, cross-checking, and open-mindedness, with the potential for either missing physics or a conceptual overhaul.
New quantum gravity theory links entropy, dark energy, and life
2 months ago
- A new theory called Gravity from Entropy (GfE) proposes that cosmic expansion can increase total entropy while decreasing entropy per unit volume, allowing local complexity to emerge.
- The theory addresses the puzzle of how galaxies, stars, planets, and life form despite the second law of thermodynamics, which requires total entropy to increase.
- GfE builds on the connection between gravity and thermodynamics, extending ideas from black hole entropy and Hawking radiation to a quantum gravity framework.
- The model reproduces general relativity at low energies but introduces a dynamically changing dark energy term at higher energies, potentially testable through observations.
- The study suggests that spacetime and gravity have an informational and thermodynamic foundation, with the quantum geometric relative entropy acting as local entropy per volume.
The age of the Universe from a large sample of the oldest Galactic stars
2 months ago
- Researchers estimated the age of the Universe using a large sample of 247,103 Milky Way stars with high-resolution spectroscopy from LAMOST DR7 and Gaia eDR3 parallaxes.
- After applying quality cuts, the final sample consisted of 155,600 stars within 5 kpc, with stellar ages estimated using YY isochrones up to 20 Gyr and cross-checked with FLAME ages from Gaia data.
- Using an MCMC reconstruction of the latent age distribution, the oldest star's age was found to be A_★ = 13.73^{+0.18}_{-0.15} Gyr, with variations due to quality cuts ranging from 13.31 to 14.02 Gyr.
- The inferred cosmic age is consistent with the 13.6 Gyr expected from CMB-calibrated ΛCDM cosmology, assuming the first long-lived stars formed when the Universe was 0.2 Gyr old.
- This agreement challenges solutions to the Hubble tension based solely on new physics before recombination, which typically predict a lower cosmic age of about 12.9 ± 0.2 Gyr.
- Stellar modeling uncertainties are unlikely to reconcile such a low age with the results, given the low metallicities of the oldest stars and independent asteroseismic constraints.

cosmology

Brief Answers to Cosmic Questions (~2000)

How Fast Is the Universe Really Expanding?

New quantum gravity theory links entropy, dark energy, and life

The age of the Universe from a large sample of the oldest Galactic stars