Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userLethal radiation from nearby supernovae helps to explain the small cosmological constant
59
0
0.0
(
0
)
Authors
Tomonori Totani
Ask ChatGPT about the research
No Arabic abstract
The observed value $Lambda_{rm obs}$ of the cosmological constant $Lambda$ is extremely smaller than theoretical expectations, and the anthropic argument has been proposed as a solution to this problem because galaxies do not form when $Lambda gg Lambda_{rm obs}$. However, the contemporary galaxy formation theory predicts that stars form even with a high value of $Lambda / Lambda_{rm obs} sim$ 50, which makes the anthropic argument less persuasive. Here we calculate the probability distribution of $Lambda$ using a model of cosmological galaxy formation, considering extinction of observers caused by radiation from nearby supernovae. The life survival probability decreases in a large $Lambda$ universe because of higher stellar density. Using a reasonable rate of lethal supernovae, we find that the mean expectation value of $Lambda$ can be close to $Lambda_{rm obs}$, and hence this effect may be essential to understand the small but nonzero value of $Lambda$. It is predicted that we are located on the edge of habitable regions about stellar density in the Galaxy, which may be tested by future exoplanet studies.
rate research
Read More
The standard concordance model of the Universe is based on the cosmological constant as the driver of accelerating expansion. This concordance model is being subjected to a growing range of inter-locking observations. In addition to using generic observational tests, one can also design tests that target the specific properties of the cosmological constant. These null tests do not rely on parametrisations of observables, but focus on quantities that are constant only if dark energy is a cosmological constant. We use supernova data in null tests that are based on the luminosity distance. In order to extract derivatives of the distance in a model-independent way, we use Gaussian Processes. We find that the concordance model is compatible with the Union 2.1 data, but the error bars are fairly large. Simulated datasets are generated for the DES supernova survey and we show that this survey will allow for a sharper null test of the cosmological constant if we assume the Universe is flat. Allowing for spatial curvature degrades the power of the null test.
Protoplanetary disk surveys by the Atacama Large Millimeter/sub-millimeter Array (ALMA) are now probing a range of environmental conditions, from low-mass star-forming regions like Lupus to massive OB clusters like $sigma$ Orionis. Here we conduct an ALMA survey of protoplanetary disks in $lambda$ Orionis, a ~5 Myr old OB cluster in Orion, with dust mass sensitivities comparable to the surveys of nearby regions (~0.4 $M_oplus$). We assess how massive OB stars impact planet formation, in particular from the supernova that may have occurred ~1 Myr ago in the core of $lambda$ Orionis; studying these effects is important as most planetary systems, including our Solar System, are likely born in cluster environments. We find that the effects of massive stars, in the form of pre-supernova feedback and/or a supernova itself, do not appear to significantly reduce the available planet-forming material otherwise expected at the evolved age of $lambda$ Orionis. We also compare a lingering massive outlier disk in $lambda$ Orionis to similar systems in other evolved regions, hypothesizing that these outliers host companions in their inner disks that suppress disk dispersal to extend the lifetimes of their outer primordial disks. We conclude with numerous avenues for future work, highlighting how $lambda$ Orionis still has much to teach us about perhaps one of the most common types of planet-forming environments in the Galaxy.
We propose a novel scenario to explain the small cosmological constant (CC) by a finely tuned inflaton potential. The tuned shape is stable under radiative corrections, and our setup is technically natural. The peculiar po- tential approximately satisfies the following conditions: the inflation is eternal if CC is positive, and not eternal if CC is negative. By introducing a slowly varying CC from a positive value to a negative value, the dominant volume of the Universe after the inflation turns out to have a vanishingly small CC. The scenario does not require eternal inflation but the e-folding number is exponentially large and the inflation scale should be low enough. The scenario can have a consistent thermal history, but the present equation of state of the Universe is predicted to differ from the prediction of the {Lambda}CDM model. A concrete model with a light scalar field is studied.
We propose a framework in which Weinbergs anthropic explanation of the cosmological constant problem also solves the hierarchy problem. The weak scale is selected by chiral dynamics that controls the stabilization of an extra dimension. When the Higgs vacuum expectation value is close to a fermion mass scale, the radius of an extra dimension becomes large, and develops an enhanced number of vacua available to scan the cosmological constant down to its observed value. At low energies, the radion necessarily appears as an unnaturally light scalar, in a range of masses and couplings accessible to fifth-force searches as well as scalar dark matter searches with atomic clocks and gravitational-wave detectors. The fermion sector that controls the size of the extra dimension consists of a pair of electroweak doublets and several singlets. These leptons satisfy approximate mass relations related to the weak scale and are accessible to the LHC and future colliders.
Progressive increases in the precision of the Hubble-constant measurement via Cepheid-calibrated Type Ia supernovae (SNe Ia) have shown a discrepancy of $sim 4.4sigma$ with the current value inferred from Planck satellite measurements of the cosmic microwave background radiation and the standard $Lambda$CDM cosmological model. This disagreement does not appear to be due to known systematic errors and may therefore be hinting at new fundamental physics. Although all of the current techniques have their own merits, further improvement in constraining the Hubble constant requires the development of as many independent methods as possible. In this work, we use SNe II as standardisable candles to obtain an independent measurement of the Hubble constant. Using 7 SNe II with host-galaxy distances measured from Cepheid variables or the tip of the red giant branch, we derive H$_0= 75.8^{+5.2}_{-4.9}$ km s$^{-1}$ Mpc$^{-1}$ (statistical errors only). Our value favours that obtained from the conventional distance ladder (Cepheids + SNe Ia) and exhibits a difference of 8.4 km s$^{-1}$ Mpc$^{-1}$ from the Planck $+Lambda$CDM value. Adding an estimate of the systematic errors (2.8 km s$^{-1}$ Mpc$^{-1}$) changes the $sim 1.7sigma$ discrepancy with Planck $+Lambda$CDM to $sim 1.4sigma$. Including the systematic errors and performing a bootstrap simulation, we confirm that the local H$_0$ value exceeds the value from the early Universe with a confidence level of 95%. As in this work we only exchange SNe II for SNe Ia to measure extragalactic distances, we demonstrate that there is no evidence that SNe Ia are the source of the H$_0$ tension.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
