1,322 research outputs found
Hubble Tension: The Evidence of New Physics
The CDM model provides a good fit to most astronomical observations
but harbors large areas of phenomenology and ignorance. With the improvements
in the precision and number of observations, discrepancies between key
cosmological parameters of this model have emerged. Among them, the most
notable tension is the 4 to 6 deviation between the Hubble
constant () estimations measured by the local distance ladder and the
cosmic microwave background (CMB) measurement. In this review, we revisit the
tension based on the latest research and sort out evidence from
solutions to this tension that might imply new physics beyond the CDM
model. The evidence leans more towards modifying the late-time universe.Comment: 39 pages, 6 figures, 3 tables, accepted for publication in Universe
(invited review for Special Issue: Advances in Astrophysics and Cosmology -
in Memory of Prof. Tan Lu
The Age-Redshift Relationship of Old Passive Galaxies
We use 32 age measurements of passively evolving galaxies as a function of
redshift to test and compare the standard model (CDM) with the Universe. We show that the latter fits the data with a reduced
for a Hubble constant km
. By comparison, the optimal flat CDM
model, with two free parameters (including and km ), fits the age-\emph{z} data with a reduced .
Based solely on their values, both models appear to account
for the data very well, though the optimized CDM parameters are only
marginally consistent with those of the concordance model ( and km ). Fitting the age-
data with the latter results in a reduced . However,
because of the different number of free parameters in these models, selection
tools, such as the Akaike, Kullback and Bayes Information Criteria, favour
over CDM with a likelihood of
versus . These results are suggestive, though not yet
compelling, given the current limited galaxy age- sample. We carry out Monte
Carlo simulations based on these current age measurements to estimate how large
the sample would have to be in order to rule out either model at a confidence level. We find that if the real cosmology is CDM, a
sample of galaxy ages would be sufficient to rule out
at this level of accuracy, while galaxy ages would be required to
rule out CDM if the real Universe were instead .Comment: 36 pages, 13 figures, 1 table; accepted for publication in The
Astronomical Journal. arXiv admin note: text overlap with arXiv:1405.238
Gamma-Ray Bursts
Gamma-ray bursts (GRB) are short and intense bursts of 100 keV1MeV
photons, usually followed by long-lasting decaying afterglow emission in a wide
range of electromagnetic wavelengths from radio to X-ray and, sometimes, even
to GeV gamma-rays. These emissions are believed to originate from a
relativistic jet, which is driven due to the collapse of special massive stars
and the mergers of compact binaries (i.e., double neutron stars or a neutron
star and a black hole). This chapter first briefly introduces the basic
observational facts of the GRB phenomena, including the prompt emission,
afterglow emission, and host galaxies. Secondly, a general theoretical
understanding of the GRB phenomena is described based on a relativistic jet's
overall dynamical evolution, including the acceleration, propagation, internal
dissipation, and deceleration phases. Here a long-lasting central engine of the
GRBs can substantially influence the dynamical evolution of the jet. In
addition, a supernova/kilonova emission can appear in the optical afterglow of
some nearby GRBs, which can provide an important probe to the nature of the GRB
progenitors. Finally, as luminous cosmological phenomena, it is expected to use
GRBs to probe the early universe and to constrain the cosmological parameters.Comment: 35 pages, 10 figures; Invited chapter for Handbook of X-ray and
Gamma-ray Astrophysics (Eds. C. Bambi and A. Santangelo, Springer Singapore,
expected in 2022
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