Shedding Light On An Undiscovered Aspect That Perfectly Mimics Cosmic Acceleration

The comparison of redshift-distance relationship for high and low-redshift supernovae revealed the surprising transition of the Universe’s expansion from deceleration to acceleration. As compared to local supernovae, remote supernovae appear 10% to 25% dimmer as they are further away than expected. The expansion rate obtained for local supernovae is higher with low redshifts as compared to the expansion rate obtained for remote supernovae with high redshifts. Since observed redshifts in an expanding Universe provide an estimate of recession velocities, therefore, it is very disturbing to find that low recession velocities (just 1% of speed of light) indicate a faster rate of expansion (acceleration), whereas high recession velocities (60% of speed of light) indicate a slower rate of expansion (deceleration). In this paper, I unravel an undiscovered aspect that perfectly mimics cosmic acceleration. Rather than “cosmic deceleration that preceded the current epoch of cosmic acceleration”, I show in this paper, that “consecutive expansion epochs of the Universe that preceded the current epoch of cosmic expansion” were responsible for placing remote supernovae further away than expected. As a consequence of consecutive expansion, expansion began for remote structures in preceding expansion epochs before it did for local structures in the current (or more recent) expansion epoch; remote supernovae, quasars, and gamma-ray bursts are therefore not only further away than expected, but they also happen to yield a slower rate of expansion, thereby suggesting their deceleration even with “superluminal expansion”. As a result of consecutive expansion, preceding expansion epochs appear to be decelerating as compared to the expansion epoch that succeeds them. The analysis is based on the redshift-distance relationship plotted for 580 type Ia supernovae from the Supernova Cosmology Project, 7 additional high-redshift type Ia supernovae discovered through the Advanced Camera for Surveys on the Hubble Space Telescope from the Great Observatories Origins Deep Survey Treasury program, and 1 additional very high-redshift type Ia supernova discovered with Wide Field and Planetary Camera 2 on the Hubble Space Telescope. The results obtained by the High-Z Supernova Search Team through observations of type Ia supernovae have also been analysed. Studies incorporating quasars and gamma-ray bursts to determine how the expansion of the Universe has changed over time have been taken into consideration as well. The results obtained in this paper have been confirmed by plotting velocity-distance relationship, expansion rate vs. time relationship, expansion factor vs. time relationship, scale factor vs. time relationship, scale factor vs. distance relationship, distance-redshift relationship, and distance modulus vs. redshift relationship, moreover, the deceleration parameter (q0) is also found to be negative (q0 < 0).

Masses of White Dwarf Binary Companions to Type Ia Supernovae Measured from Runaway Velocities

The recently proposed “dynamically driven double-degenerate double-detonation” (D6) scenario posits that Type Ia supernovae (SNe) may occur during dynamically unstable mass transfer between two white dwarfs (WDs) in a binary. This scenario predicts that the donor WD may then survive the explosion and be released as a hypervelocity runaway, opening up the exciting possibility of identifying remnant stars from D6 SNe and using them to study the physics of detonations that produce Type Ia SNe. Three candidate D6 runaway objects have been identified in Gaia data. The observable runaway velocity of these remnant objects represents their orbital speed at the time of SN detonation. The orbital dynamics and Roche lobe geometry required in the D6 scenario place specific constraints on the radius and mass of the donor WD that becomes the hypervelocity runaway. In this Letter, we calculate the radii required for D6 donor WDs as a function of the runaway velocity. Using mass–radius relations for WDs, we then constrain the masses of the donor stars as well. With measured velocities for each of the three D6 candidate objects based on Gaia EDR3, this work provides a new probe of the masses and mass ratios in WD binary systems that produce SN detonations and hypervelocity runaways.

ParSNIP: Generative Models of Transient Light Curves with Physics-enabled Deep Learning

We present a novel method to produce empirical generative models of all kinds of astronomical transients from data sets of unlabeled light curves. Our hybrid model, which we call ParSNIP, uses a neural network to model the unknown intrinsic diversity of different transients and an explicit physics-based model of how light from the transient propagates through the universe and is observed. The ParSNIP model predicts the time-varying spectra of transients despite only being trained on photometric observations. With a three-dimensional intrinsic model, we are able to fit out-of-sample multiband light curves of many different kinds of transients with model uncertainties of 0.04–0.06 mag. The representation learned by the ParSNIP model is invariant to redshift, so it can be used to perform photometric classification of transients even with heavily biased training sets. Our classification techniques significantly outperform state-of-the-art methods on both simulated (PLAsTiCC) and real (PS1) data sets with 2.3× and 2× less contamination, respectively, for classification of Type Ia supernovae. We demonstrate how our model can identify previously unobserved kinds of transients and produce a sample that is 90% pure. The ParSNIP model can also estimate distances to Type Ia supernovae in the PS1 data set with an rms of 0.150 ± 0.007 mag compared to 0.155 ± 0.008 mag for the SALT2 model on the same sample. We discuss how our model could be used to produce distance estimates for supernova cosmology without the need for explicit classification.

The Foundation Supernova Survey: Photospheric Velocity Correlations in Type Ia Supernovae

The ejecta velocities of Type Ia supernovae (SNe Ia), as measured by the Si ii λ6355 line, have been shown to correlate with other supernova properties, including color and standardized luminosity. We investigate these results using the Foundation Supernova Survey, with a spectroscopic data release presented here, and photometry analyzed with the SALT2 light-curve fitter. We find that the Foundation data do not show significant evidence for an offset in color between SNe Ia with high and normal photospheric velocities, with Δc = 0.004 ± 0.015. Our SALT2 analysis does show evidence for redder high-velocity SNe Ia in other samples, including objects from the Carnegie Supernova Project, with a combined sample yielding Δc = 0.018 ± 0.008. When split on velocity, the Foundation SNe Ia also do not show a significant difference in Hubble diagram residual, ΔHR = 0.015 ± 0.049 mag. Intriguingly, we find that SN Ia ejecta velocity information may be gleaned from photometry, particularly in redder optical bands. For high-redshift SNe Ia, these rest-frame red wavelengths will be observed by the Nancy Grace Roman Space Telescope. Our results are in line with previous work that suggests SN Ia host-galaxy stellar mass is correlated with ejecta velocity: high-velocity SNe Ia are found nearly exclusively in high-stellar-mass hosts. However, host-galaxy properties alone do not explain velocity-dependent differences in supernova colors and luminosities across samples. Measuring and understanding the connection between intrinsic explosion properties and supernova environments, across cosmic time, will be important for precision cosmology with SNe Ia.

Carnegie Supernova Project: The First Homogeneous Sample of Super-Chandrasekhar-mass/2003fg-like Type Ia Supernovae

We present a multiwavelength photometric and spectroscopic analysis of 13 super-Chandrasekhar-mass/2003fg-like Type Ia supernovae (SNe Ia). Nine of these objects were observed by the Carnegie Supernova Project. The 2003fg-like SNe have slowly declining light curves (Δm 15(B) < 1.3 mag), and peak absolute B-band magnitudes of −19 < M B < −21 mag. Many of the 2003fg-like SNe are located in the same part of the luminosity–width relation as normal SNe Ia. In the optical B and V bands, the 2003fg-like SNe look like normal SNe Ia, but at redder wavelengths they diverge. Unlike other luminous SNe Ia, the 2003fg-like SNe generally have only one i-band maximum, which peaks after the epoch of the B-band maximum, while their near-IR (NIR) light-curve rise times can be ≳40 days longer than those of normal SNe Ia. They are also at least 1 mag brighter in the NIR bands than normal SNe Ia, peaking above M H = −19 mag, and generally have negative Hubble residuals, which may be the cause of some systematics in dark-energy experiments. Spectroscopically, the 2003fg-like SNe exhibit peculiarities such as unburnt carbon well past maximum light, a large spread (8000–12,000 km s−1) in Si ii λ6355 velocities at maximum light with no rapid early velocity decline, and no clear H-band break at +10 days. We find that SNe with a larger pseudo-equivalent width of C ii at maximum light have lower Si ii λ6355 velocities and more slowly declining light curves. There are also multiple factors that contribute to the peak luminosity of 2003fg-like SNe. The explosion of a C–O degenerate core inside a carbon-rich envelope is consistent with these observations. Such a configuration may come from the core-degenerate scenario.

Effect of Cosmic Mean Metallicity on the Supernovae Cosmology

The initial metallicity of Type Ia Supernovae (SNe Ia) progenitor that is increasing with the cosmological chemical evolution will directly lead to a decrease of the 56Ni formed during the nucleosynthesis and then a varying standard candle. The variation may seriously affect our understanding of the evolving universe. In this work, we derived the relationships between 56Ni yield and metallicity in different progenitor channels. The evolution of the cosmic mean metallicity (CMM) was used to estimate the initial metallicity of progenitors. The effect of the delay times from the birth of progenitors to their explosion was also considered. The corrections of SNe Ia luminosity were estimated and the influences of the different progenitor channels and CMM evolution rates were examined. Several important cosmological parameters were updated according to the luminosity corrections.

HD 74438: A tight spectroscopic quadruple as possible progenitor of sub-Chandrasekhar Type Ia supernovae

Stars often form in multiple systems and may follow a complex evolution involving mass transfer and collisions, leading to mergers that are possible progenitors of Type Ia supernovae (SNe) [1, 2]. The progenitors of such explosions are still highly debated [3]. While binaries have received much attention so far, higher-order stellar systems show a wide variety of interactions especially in tight systems, like long-term gravitational effects playing a key role in triple (where they are called von Zeipel-Lidov-Kozai , [4, 5], hereafter ZLK, oscillations) and quadruple systems. Here we report on the properties of the first spectroscopic quadruple (SB4) found within a star cluster: the 2+2 hierarchical system HD 74438 [6]. Its membership in the open cluster IC 2391 makes it the youngest (43 My) SB4 discovered so far. The eccentricity of the 6 y outer period is 0.46 and the two inner orbits, with periods of 20.5 d and 4.4 d, and eccentricities of 0.36 and 0.15, are not coplanar. Using an innovative combination of ground-based high resolution spectroscopy [7, 8, 9, 10] and Gaia/Hipparcos astrometry [11, 12, 13, 14], we show that this system is undergoing secular interaction that likely pumped the eccentricity of one of the inner orbit higher than expected for the spectral types of its components. We compute the future evolution of HD 74438 by considering gravitational dynamics, stellar evolution, and binary interactions [15], and show that this system is an excellent candidate progenitor of sub-Chandrasekhar Type Ia supernova through white dwarf (WD) mergers. This specific type of SNIa better accounts for the chemical evolution of iron-peak elements in the Galaxy [16].

Circumstellar Medium Constraints on the Environment of Two Nearby Type Ia Supernovae: SN 2017cbv and SN 2020nlb

We present deep Chandra X-ray observations of two nearby Type Ia supernovae, SN 2017cbv and SN 2020nlb, which reveal no X-ray emission down to a luminosity L X ≲ 5.3 × 1037 and ≲ 5.4 × 1037 erg s−1 (0.3–10 keV), respectively, at ∼16–18 days after the explosion. With these limits, we constrain the pre-explosion mass-loss rate of the progenitor system to be M ̇ < 7.2 × 10−9 and < 9.7 × 10−9 M ⊙ yr−1 for each (at a wind velocity v w = 100 km s−1 and a radius of R ≈ 1016 cm), assuming any X-ray emission would originate from inverse Compton emission from optical photons upscattered by the supernova shock. If the supernova environment was a constant-density medium, we would find a number density limit of n CSM < 36 and < 65 cm−3, respectively. These X-ray limits rule out all plausible symbiotic progenitor systems, as well as large swathes of parameter space associated with the single degenerate scenario, such as mass loss at the outer Lagrange point and accretion winds. We also present late-time optical spectroscopy of SN 2020nlb, and set strong limits on any swept up hydrogen (L Hα < 2.7 × 1037 erg s−1) and helium (L He,λ6678 < 2.7 × 1037 erg s−1) from a nondegenerate companion, corresponding to M H ≲ 0.7–2 × 10−3 M ⊙ and M He ≲ 4 × 10−3 M ⊙. Radio observations of SN 2020nlb at 14.6 days after explosion also yield a non-detection, ruling out most plausible symbiotic progenitor systems. While we have doubled the sample of normal Type Ia supernovae with deep X-ray limits, more observations are needed to sample the full range of luminosities and subtypes of these explosions, and set statistical constraints on their circumbinary environments.