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Archive for the ‘COSMOLOGY’ Category

How Friedmann Shod Einstein

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Vladimir O. Soloviev
Exactly one hundred years ago, Alexander Friedmann discovered that General Relativity (GR) predicts non-stationary Universe. His equation describing the evolution of the Universe is now named after him. In this paper we briefly recall the human and scientific aspects of this revolutionary change in our picture of the world. We also recall that immediately after finishing his seminal paper Friedmann wrote a book on GR for a wide audience. In that book he presented an impressive picture of the expanding Universe created from nothing…

Read more at https://arxiv.org/pdf/2204.10650.pdf

Written by physicsgg

April 25, 2022 at 7:53 am

Posted in COSMOLOGY, RELATIVITY

The Genesis of the No-Boundary Wave Function of the Universe

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James Hartle
Brief recollections by the author about how he and Stephen Hawking arrived at the theory of the No Boundary Quantum State of the Universe. Read more at

Click to access 2202.07020.pdf

Written by physicsgg

February 16, 2022 at 12:42 pm

Posted in COSMOLOGY

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String Theory or Loop Quantum Gravity?

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David Gross vs Carlo Rovelli

String theory has dominated discussions at the frontiers of physics for decades, especially in the attempts to build a quantum theory of gravity. But does it deserve its exalted status? Nobel Prize winner and String Theory pioneer David Gross debates Carlo Rovelli, one of the founding fathers of Loop Quantum Gravity. The discussion is lively; full of insights, insults, backhanded compliments and even some common ground as to the nature of physics.

Timecodes 0:00 Introduction 3:00 David Gross early years, 4:00 Carlo Rovelli early years 5:22 David on string theory 20:22 Carlo on string theory 31:08 David&Carlo on string theory 53:20 Loop Quantum Gravity 1:00 David&Carlo on LQG 1:21 Black Holes 1:30 Predictions and the Scientific Method

Written by physicsgg

December 23, 2021 at 7:12 pm

Posted in COSMOLOGY

Three Impossible Theories

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Leonard Susskind
I will begin by conjecturing a cosmological generalization of black hole complementarity (also known as the central dogma). I will then discuss three theories and argue that they are inconsistent with second law of thermodynamics if the cosmological version of the dogma is correct. The three theories are: the big rip; cyclic cosmology; and the Farhi-Guth-Guven mechanism for creating inflating universes behind black hole horizons. … Read more at https://arxiv.org/abs/2107.11688

Written by physicsgg

September 21, 2021 at 8:59 am

Posted in COSMOLOGY

Juan Maldacena: Black Holes, Hawking Radiation and the Nature of Spacetime

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Written by physicsgg

April 23, 2021 at 3:27 pm

Posted in COSMOLOGY

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Dark Energy: is it ‘just’ Einstein’s Cosmological Constant Λ?

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Ofer Lahav
The Cosmological Constant Lambda, a concept introduced by Einstein in 1917, has been with us ever since in different variants and incarnations, including the broader concept of Dark Energy. Current observations are consistent with a value of Lambda corresponding to about present-epoch 70% of the critical density of the Universe. This is causing the speeding up (acceleration) of the expansion of the Universe over the past 6 billion years, a discovery recognised by the 2011 Nobel Prize in Physics. Coupled with the flatness of the Universe and the amount of 30% matter (5% baryonic and 25% Cold Dark Matter), this forms the so-called Lambda-CDM standard model, which has survived many observational tests over about 30 years. However, there are currently indications of inconsistencies (`tensions’ ) within Lambda-CDM on different values of the Hubble Constant and the clumpiness factor. Also, time variation of Dark Energy and slight deviations from General Relativity are not ruled out yet. Several grand projects are underway to test Lambda-CDM further and to estimate the cosmological parameters to sub-percent level. If Lambda-CDM will remain the standard model, then the ball is back in the theoreticians’ court, to explain the physical meaning of Lambda. Is Lambda an alteration to the geometry of the Universe, or the energy of the vacuum? Or maybe it is something different, that manifests a yet unknown higher-level theory?

read more at https://arxiv.org/abs/2009.10177

Click to access 2009.10177.pdf

Written by physicsgg

September 25, 2020 at 11:26 pm

Posted in COSMOLOGY, DARK ENERGY

Profiles of James Peebles, Michel Mayor, and Didier Queloz: 2019 Nobel Laureates in Physics

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Neta Bahcall, Adam Burrows

Published in PNAS, 117, 2, 799 – 801 (January 2020)

Mankind has long been fascinated by the mysteries of our Universe: How old and how big is the
Universe? How did the Universe begin and how is it evolving? What is the composition of the
Universe and the nature of its dark-matter and dark-energy? What is our Earth’s place in the cosmos
and are there other planets (and life) around other stars?

The 2019 Nobel Prize in Physics honors three pioneering scientists for their fundamental contributions to basic cosmic questions – Professor James Peebles (Princeton University), Michel Mayor (University of Geneva), and Didier Queloz (University of Geneva and the University of Cambridge) – “for contributions to our understanding of the evolution of the universe and Earth’s place in the cosmos,” with one half to James Peebles “for theoretical discoveries in physical cosmology,” and the other half jointly to Michel Mayor and Didier Queloz “for the discovery of an exoplanet orbiting a solar-type star.” We summarize the historical and scientific backdrop to this year’s Physics Nobel.

Read more at https://arxiv.org/ftp/arxiv/papers/2001/2001.08511.pdf

Written by physicsgg

January 24, 2020 at 5:40 pm

Posted in ASTRONOMY, ASTROPHYSICS, COSMOLOGY

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New evidence shows that the key assumption made in the discovery of dark energy is in error

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High precision age dating of supernova host galaxies reveals that the luminosity evolution of supernovae is significant enough to question the very existence of dark energy

Figure 1. Luminosity evolution mimicking dark energy in supernova (SN) cosmology. The Hubble residual is the difference in SN luminosity with respect to the cosmological model without dark energy (the black dotted line). The cyan circles are the binned SN data from Betoule et al. (2014). The red line is the evolution curve based on our age dating of early-type host galaxies. The comparison of our evolution curve with SN data shows that the luminosity evolution can mimic Hubble residuals used in the discovery and inference of the dark energy (the black solid line).

The most direct and strongest evidence for the accelerating universe with dark energy is provided by the distance measurements using type Ia supernovae (SN Ia) for the galaxies at high redshift. This result is based on the assumption that the corrected luminosity of SN Ia through the empirical standardization would not evolve with redshift.

New observations and analysis made by a team of astronomers at Yonsei University (Seoul, South Korea), together with their collaborators at Lyon University and KASI, show, however, that this key assumption is most likely in error. The team has performed very high-quality (signal-to-noise ratio ~175) spectroscopic observations to cover most of the reported nearby early-type host galaxies of SN Ia, from which they obtained the most direct and reliable measurements of population ages for these host galaxies. They find a significant correlation between SN luminosity and stellar population age at a 99.5% confidence level. As such, this is the most direct and stringent test ever made for the luminosity evolution of SN Ia. Since SN progenitors in host galaxies are getting younger with redshift (look-back time), this result inevitably indicates a serious systematic bias with redshift in SN cosmology. Taken at face values, the luminosity evolution of SN is significant enough to question the very existence of dark energy. When the luminosity evolution of SN is properly taken into account, the team found that the evidence for the existence of dark energy simply goes away (see Figure 1).

Commenting on the result, Prof. Young-Wook Lee (Yonsei Univ., Seoul) who was leading the project said; “Quoting Carl Sagan, extraordinary claims require extraordinary evidence, but I am not sure we have such extraordinary evidence for dark energy. Our result illustrates that dark energy from SN cosmology, which led to the 2011 Nobel Prize in Physics, might be an artifact of a fragile and false assumption”.

Other cosmological probes, such as CMB (Cosmic Microwave Background) and BAO (Baryonic Acoustic Oscillations), are also known to provide some indirect and “circumstantial” evidence for dark energy, but it was recently suggested that CMB from Planck mission no longer supports the concordance cosmological model which may require new physics (Di Valentino, Melchiorri, & Silk 2019). Some investigators have also shown that BAO and other low-redshift cosmological probes can be consistent with a non-accelerating universe without dark energy (see, for example, Tutusaus et al. 2017). In this respect, the present result showing the luminosity evolution mimicking dark energy in SN cosmology is crucial and is very timely.

This result is reminiscent of the famous Tinsley-Sandage debate in the 1970s on luminosity evolution in observational cosmology, which led to the termination of the Sandage project originally designed to determine the fate of the universe.

Read more at https://astro.yonsei.ac.kr/galaxy/galaxy01/research.do?mode=view&articleNo=78249 and https://arxiv.org/abs/1912.04903

Written by physicsgg

January 8, 2020 at 6:04 pm