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

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

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

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


Is the expansion of the universe accelerating? All signs still point to yes

David Rubin, Jessica Heitlauf
Type Ia supernovae (SNe Ia) provided the first strong evidence that the expansion of the universe is accelerating. With SN samples now more than ten times larger than those used for the original discovery and joined by other cosmological probes, this discovery is on even firmer ground. Two recent, related studies (Nielsen et al. 2016 and Colin et al. 2019, hereafter N16 and C19, respectively) have claimed to undermine the statistical significance of the SN Ia constraints. Rubin & Hayden (2016) (hereafter RH16) showed N16 made an incorrect assumption about the distributions of SN Ia light-curve parameters, while C19 also fails to remove the impact of the motion of the solar system from the SN redshifts, interpreting the resulting errors as evidence of a dipole in the deceleration parameter. Building on RH16, we outline the errors C19 makes in their treatment of the data and inference on cosmological parameters. Reproducing the C19 analysis with our proposed fixes, we find that the dipole parameters have little effect on the inferred cosmological parameters. We thus affirm the conclusion of RH16: the evidence for acceleration is secure.

Read more at https://arxiv.org/abs/1912.02191

Read also: “No Dark Energy? No Chance, Cosmologists Contend


Exploring evidence of interaction between dark energy and dark matter

One of the most important problems of theoretical physics is to explain the fact that the universe is in a phase of accelerated expansion. Since 1998 the physical origin of cosmic acceleration remains a deep mystery. According to general relativity (GR), if the universe is filled with ordinary matter or radiation, the two known constituents of the universe, gravity should slow the expansion. Since the expansion is speeding up, we are faced with two possibilities, either of which would have profound implications for our understanding of the cosmos and of the laws of physics. The first is that 75% of the energy density of the universe exists in a new form with large negative pressure, called dark energy (DE). The other possibility is that GR breaks down on cosmological scales and must be replaced with a more complete theory of gravity. In this paper we consider the first option. The cosmological constant, the simplest explanation of accelerated expansion, has a checkered history having been invoked and subsequently withdrawn several times before. In quantum field theory, we estimate the value of the cosmological constant as the zero-point energy with a short-cut scale, for example the Planck scale, which results in an excessively greater value than the observational results….

read more at https://arxiv.org/pdf/1804.03296.pdf


The dark side of cosmology: Dark matter and dark energy

The multiple components that compose our universe. Dark energy comprises 69% of the mass energy density of the universe, dark matter comprises 25%, and “ordinary” atomic matter makes up 5%. There are other observable subdominant components: Three different types of neutrinos comprise at least 0.1%, the cosmic background radiation makes up 0.01%, and black holes comprise at least 0.005%.

The multiple components that compose our universe.
Dark energy comprises 69% of the mass energy density of the universe, dark matter comprises 25%, and “ordinary” atomic matter makes up 5%. There are other observable subdominant components: Three different types of neutrinos comprise at least 0.1%, the cosmic background radiation makes up 0.01%, and black holes comprise at least 0.005%.

A simple model with only six parameters (the age of the universe, the density of atoms, the density of matter, the amplitude of the initial fluctuations, the scale dependence of this amplitude, and the epoch of first star formation) fits all of our cosmological data . Although simple, this standard model is strange. The model implies that most of the matter in our Galaxy is in the form of “dark matter,” a new type of particle not yet detected in the laboratory, and most of the energy in the universe is in the form of “dark energy,” energy associated with empty space. Both dark matter and dark energy require extensions to our current understanding of particle physics or point toward a breakdown of general relativity on cosmological scales…
…Read more at http://www.sciencemag.org/content/347/6226/1100.full


What is dark energy?

Sure, the Universe is expanding, and that expansion is accelerating. But beyond simply calling the cause “dark energy,” what do we know about it?

“I must choose between despair and Energy —— I choose the latter.” -John Keats

All week long, some of you have been racking your brains to come up with the deepest, most mysterious questions about the Universe to highlight for our Ask Ethan column. We’ve gotten some outstanding questions and suggestions that you’ve sent in, and while it’s a pity I can only choose one, this week’s honor goes to Piyush Gupta, who asks:

[We] have found that dark energy makes up about 70% [of the] energy in the universe. We have evidence of dark energy from multiple observations. It has [a] real effect on the evolution of [the] observable universe. But what is dark energy? Do we have any idea? Do we have any good models for it?

As it turns out, we do have some good ideas, but let’s make sure we’re all on the same page first….

…. Read more at https://medium.com/starts-with-a-bang/ask-ethan-58-what-is-dark-energy-61db04945b3d