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Cepheids have been the cornerstone of the extragalactic distance scale for a century. With high-quality data, these luminous supergiants exhibit a small dispersion in their Leavitt (period–luminosity) relation, particularly at longer wavelengths, and few methods rival the precision possible with Cepheid distances. In these proceedings, we present an overview of major observational programs pertaining to the Cepheid extragalactic distance scale, its progress and remaining challenges. In addition, we present preliminary new results on Cepheids from the James Webb Space Telescope (JWST). The launch of JWST has opened a new chapter in the measurement of extragalactic distances and the Hubble constant. JWST offers a resolution three times that of the Hubble Space Telescope (HST) with nearly 10 times the sensitivity. It has been suggested that the discrepancy in the value of the Hubble constant based on Cepheids compared to that inferred from measurements of the cosmic microwave background requires new and additional physics beyond the standard cosmological model. JWST observations will be critical in reducing remaining systematics in the Cepheid measurements and for confirming if new physics is indeed required. Early JWST data for the galaxy, NGC 7250 show a decrease in scatter in the Cepheid Leavitt law by a factor of two relative to existing HST data and demonstrate that crowding/blending effects are a significant issue in a galaxy as close as 20 Mpc. [ABSTRACT FROM AUTHOR]

Using parallaxes from Gaia Early Data Release 3 (EDR3), we determine multi-wavelength BVI c , JHK s , and [3.6] and [4.5] micron absolute magnitudes for 37 nearby Milky Way Cepheids, covering the period range between 5 and 60 days. We apply these periodâ€"luminosity relations to Cepheids in the Large and Small Magellanic Clouds and find that the derived distances are significantly discrepant with the geometric distances according to detached eclipsing binaries (DEBs). We explore several potential causes of these issues, including reddening, metallicity, and the existence of an additional zero-point offset, but none provide a sufficient reconciliation with both DEB distances. We conclude that the combination of the systematic uncertainties on the EDR3 parallaxes with the uncertainties on the effect of metallicity on the Cepheid distance scale leads to a systematic error floor of approximately 3%. We therefore find that the EDR3 data are not sufficiently accurate in the regime of these bright Cepheids to determine extragalactic distances precise to the 1% level at this time, in agreement with a number of contemporary studies. [ABSTRACT FROM AUTHOR]

Measurement of the distances to nearby galaxies has improved rapidly in recent decades. The ever-present challenge is to reduce systematic effects, especially as greater distances are probed and the uncertainties become larger. In this paper, we combine several recent calibrations of the tip of the red giant branch (TRGB) method. These calibrations are internally self-consistent at the 1% level. New Gaia Early Data Release 3 data provide an additional consistency check at a (lower) 5% level of accuracy, a result of the well-documented Gaia angular covariance bias. The updated TRGB calibration applied to a sample of Type Ia supernovae from the Carnegie Supernova Project results in a value of the Hubble constant of H0 = 69.8 ± 0.6 (stat) ± 1.6 (sys) km s−1 Mpc−1. No statistically significant difference is found between the value of H0 based on the TRGB and that determined from the cosmic microwave background. The TRGB results are also consistent to within 2σ with the SHoES and Spitzer plus Hubble Space Telescope (HST) Key Project Cepheid calibrations. The TRGB results alone do not demand additional new physics beyond the standard (ΛCDM) cosmological model. They have the advantage of simplicity of the underlying physics (the core He flash) and small systematic uncertainties (from extinction, metallicity, and crowding). Finally, the strengths and weaknesses of both the TRGB and Cepheids are reviewed, and prospects for addressing the current discrepancy with future Gaia, HST, and James Webb Space Telescope observations are discussed. Resolving this discrepancy is essential for ascertaining if the claimed tension in H0 between the locally measured and CMB-inferred values is physically motivated. [ABSTRACT FROM AUTHOR]

The local determination of the Hubble constant sits at a crossroad. Current estimates of the local expansion rate of the universe differ by about 1.7σ, derived from the Cepheid- and TRGB-based calibrations, applied to Type Ia supernovae. To help elucidate possible sources of systematic error causing the tension, we show in this study the recently developed distance indicator, the J-region Asymptotic Giant Branch (JAGB) method, can serve as an independent cross-check and comparison with other local distance indicators. Furthermore, we make the case that the JAGB method has substantial potential as an independent, precise, and accurate calibrator of Type Ia supernovae for the determination of H0. Using the Local Group galaxy Wolf–Lundmark–Melotte (WLM), we present distance comparisons between the JAGB method, a TRGB measurement at near-infrared (JHK) wavelengths, a TRGB measurement in the optical I band, and a multiwavelength Cepheid period–luminosity relation determination. We find μ 0 (JAGB) = 24.97 ± 0.02 (stat) ± 0.04 (sys) mag μ 0 (TRGB NIR) = 24.98 ± 0.04 (stat) ± 0.07 (sys) mag μ 0 (TRGB F 814 W) = 24.93 ± 0.02 (stat) ± 0.06 (sys) mag μ 0 (Cepheids) = 24.98 ± 0.03 (stat) ± 0.04 (sys) mag. All four methods are in good agreement, confirming the local self-consistency of the four distance scales at the 3% level and adding confidence that the JAGB method is as accurate and as precise a distance indicator as either of the other three astrophysically based methods. [ABSTRACT FROM AUTHOR]

We present photometric and spectroscopic observations of SN 2013aa and SN 2017cbv, two nearly identical type Ia supernovae (SNe Ia) in the host galaxy NGC 5643. The optical photometry has been obtained using the same telescope and instruments used by the Carnegie Supernova Project. This eliminates most instrumental systematics and provides light curves in a stable and well-understood photometric system. Having the same host galaxy also eliminates systematics due to distance and peculiar velocity, providing an opportunity to directly test the relative precision of SNe Ia as standard candles. The two SNe have nearly identical decline rates, negligible reddenings, and remarkably similar spectra, and, at a distance of ∼20 Mpc, they are ideal potential calibrators for the absolute distance using primary indicators such as Cepheid variables. We discuss to what extent these two SNe can be considered twins and compare them with other supernova "siblings" in the literature and their likely progenitor scenarios. Using 12 galaxies that hosted two or more SNe Ia, we find that when using SNe Ia, and after accounting for all sources of observational error, one gets consistency in distance to 3%. [ABSTRACT FROM AUTHOR]

We compare mid-infrared (IR) 3.6 and 4.5 µm Warm Spitzer observations for Cepheids in the Milky Way and the Large and Small Magellanic Clouds. Using models, we explore in detail the effect of the CO rotation-vibration band-head at 4.6 µmon the mid-IR photometry. We confirm the temperature sensitivity of the CO band-head at 4.6 µm and find no evidence for an effect at 3.6 µm.We compare the ([3.6]-[4.5]) period-colour relations in the MW, LMC and SMC. The slopes of the period-colour relations for the three galaxies are in good agreement, but there is a trend in zero-point with metallicity, with the lowest metallicity Cepheids having redder mid-IR colours. Finally, we present a colour-[Fe/H] relation based on published spectroscopic metallicities. This empirical relation, calibrated to the metallicity system of Genovali et al., demonstrates that the ([3.6]-[4.5]) colour provides a reliable metallicity indicator for Cepheids, with a precision comparable to current spectroscopic determinations. [ABSTRACT FROM AUTHOR]

The impact of metallicity on the Cepheid period-luminosity (P – L) relation is investigated using Hubble Space Telescope Advanced Camera for Surveys V and I images of M101. Variations in the reddening-free Wesenheit parameter (W), which is employed as a proxy for luminosity, are examined as a function of the radial distance from the center of M101 (and thus metallicity). We determine that there is no dependence of the slope on metallicity. However, the intercept is found to depend on metallicity by γVI = –0.33 ± 0.12 mag dex–1 and γVI = –0.71 ± 0.17 mag dex–1 using 2σ and 3σ rejection criteria, respectively. Sigma-clipping impacts the derived metallicity dependence, and the 2σ criterion applied likely mitigates blending, particularly in the crowded inner regions of M101. A metallicity-corrected distance for M101 is obtained from 619 Cepheids (μ = 28.96 ± 0.11), a result that agrees with the recently determined SN Ia distance. The metallicity effects described can be bypassed by working at near and mid-infrared wavelengths (e.g., the Carnegie Hubble Program). [ABSTRACT FROM AUTHOR]

We have observed the dwarf galaxy IC 1613, at multiple epochs in the mid-infrared using Spitzer and contemporaneously in the near-infrared using the new FourStar near-infrared camera on Magellan. We have constructed Cepheid period–luminosity relations in the J, H, Ks, [3.6] and [4.5] bands and have used the run of their apparent distance moduli as a function of wavelength to derive the line-of-sight reddening and distance to IC 1613. Using a nine-band fit, we find E(B − V ) = 0.05 ± 0.01 mag and an extinction-corrected distance modulus of μ0 = 24.29 ± 0.03statistical ± 0.03systematic mag. By comparing our multi-band and [3.6] distance moduli to results from the tip of the red giant branch and red clump distance indicators, we find that metallicity has no measurable effect on Cepheid distances at 3.6μm in the metallicity range −1.0 ⩽ [Fe/H] ⩽ 0.2, hence derivations of the Hubble constant at this wavelength require no correction for metallicity. [ABSTRACT FROM AUTHOR]

Twenty years ago, there was disagreement at a level of a factor of two as regards the value of the expansion rate of the Universe. Ten years ago, a value that was good to 10% was established using the Hubble Space Telescope (HST), completing one of the primary missions that NASA designed and built the HST to undertake. Today, after confronting most of the systematic uncertainties listed at the end of the Key Project, we are looking at a value of the Hubble constant that is plausibly known to within 3%. In the near future, an independently determined value of H0 good to 1% is desirable to constrain the extraction of other cosmological parameters from the power spectrum of the cosmic microwave background in defining a concordance model of cosmology. We review recent progress and assess the future prospects for those tighter constraints on the Hubble constant, which were unimaginable just a decade ago. [ABSTRACT FROM PUBLISHER]

The Carnegie Hubble Program is designed to improve the extragalactic distance scale using data from the postcryogenic era of Spitzer. The ultimate goal is a determination of the Hubble constant to an accuracy of 2%. This paper is the first in a series on the Cepheid population of the Large Magellanic Cloud, and focusses on the period-luminosity (PL) relations (Leavitt laws) that will be used, in conjunction with observations of Milky Way Cepheids, to set the slope and zero point of the Cepheid distance scale in the mid-infrared. To this end, we have obtained uniformly sampled light curves for 85 LMC Cepheids, having periods between 6 and 140 days. PL and period-color relations are presented in the 3.6 µm and 4.5 µm bands. We demonstrate that the 3.6/zm band is a superb distance indicator. The cyclical variation of the [3.6]-[4.5] color has been measured for the first time. We attribute the anaplitude and phase of the color curves to the dissociation and recombination of CO molecules in the Cepheid's atmosphere. The CO affects only the 4.5 µm flux making it a potential metallicity indicator. [ABSTRACT FROM AUTHOR]

We undertake a new test of the metaflicity sensitivity of the Leavitt Law for classical Cepheids. We derive an empirical calibration of the apparent luminosities of Cepheids as measured from the optical through the mid- infrared (0.45-8.0 μm) as a function of spectroscopic [Fe/HI abundances of individual Cepheids in the Large Magellanic Cloud (LMC) from Romaniello et al. The cumulative trend over the entire wavelength range shows a nearly monotonic behavior. The sense of the trend is consistent with differential line blanketing in the optical, leading to stars of high metallicity being fainter in the optical. This is followed by a reversal in the trend at longer wavelengths, with the crossover occurring near the K band at about 2.2 μm, consistent with a subsequent redistribution of energy resulting in a mild brightening of Cepheids (with increased metallicity) at mid-infrared wavelengths. This conclusion agrees with that of Romaniello et al. based on a differential comparison of the mean V- and K-band Leavitt Laws for the Galaxy, Small Magellanic Cloud, and LMC, but is opposite in sign to most other empirical tests of the sensitivity of Cepheid distances to mean [O/H] H II region abundances. We also search for a correlation of Cepheid host-galaxy metallicity with deviations of the galaxy's Cepheid distance from that predicted from a pure Hubble flow. Based on Cepheid distances to 26 nearby galaxies in the local flow, only a very weak signal is detected giving δμ0 = -0.17(±0.31)([O/H] - 8.80) - 0.21(±0.10). This is in agreement with previous determinations, but statistically inconclusive. [ABSTRACT FROM AUTHOR]

Considerable progress has been made in determining the Hubble constant over the past two decades. We discuss the cosmological context and importance of an accurate measurement of the Hubble constant, focusing on six high-precision distance-determination methods: Cepheids, tip of the red giant branch, maser galaxies, surface brightness fluctuations, the Tully-Fisher relation, and Type Ia supernovae. We discuss in detail known systematic errors in the measurement of galaxy distances and how to minimize them. Our best current estimate of the Hubble constant is 73±2 (random) ± 4 (systematic) km s−1 Mpc−1. The importance of improved accuracy in the Hubble constant will increase over the next decade with new missions and experiments designed to increase the precision in other cosmological parameters. We outline the steps that will be required to deliver a value of the Hubble constant to 2% systematic uncertainty and discuss the constraints on other cosmological parameters that will then be possible with such accuracy. [ABSTRACT FROM AUTHOR]

Presents research on the distance to the galaxy NGC4258 from observation of Cepheid variable stars. How the pulsation of the variable stars make them useful as standards for determining distances to galaxies; Implications for the expansion age of the Universe.

Focuses on the measurement of the distance of the Virgo cluster of galaxies using the Hubble Space Telescope. Determination of the present expansion rate of the universe or Hubble constant; Specification of the evolution of the universe through the Friedmann equation; Measurement of the distance of galaxies through using the observation of Cepheids.

Reports on a Cepheid distance of 18.6 +/- 1.9 +/- Mpc for the galaxy NGC1365 in Fornax, a cluster of galaxies in the Southern Hemisphere. How both galaxy distances and velocities are required in order to determine the expansion rate of the universe; Estimates; Findings.

We present a revised distance to the nearby galaxy NGC 6822 using a new multi-band fit to both previously published and new optical, near-, and mid-infrared data for Cepheid variables. The new data presented in this study include multi-epoch observations obtained in 3.6 μm and 4.5 μm with the Spitzer Space Telescope taken for the Carnegie Hubble Program. We also present new observations in J, H, and Ks with FourStar on the Magellan Baade Telescope at Las Campanas Observatory. We determine mean magnitudes and present new period-luminosity relations in V, I, J, H, Ks, Infrared Array Camera 3.6 μm, and 4.5 μm. In addition to using the multi-band distance moduli to calculate extinction and a true distance, we present a new method for determining an extinction-corrected distance modulus from multi-band data with varying sample sizes. We combine the distance moduli and extinction for individual stars to determine E(B – V) = 0.35 ± 0.04 and a true distance modulus μo = 23.38 ± 0.02stat ± 0.04sys. [ABSTRACT FROM AUTHOR]

Using a mid-infrared calibration of the Cepheid distance scale based on recent observations at 3.6 μm with the Spitzer Space Telescope, we have obtained a new, high-accuracy calibration of the Hubble constant. We have established the mid-IR zero point of the Leavitt law (the Cepheid period-luminosity relation) using time-averaged 3.6 μm data for 10 high-metallicity, Milky Way Cepheids having independently measured trigonometric parallaxes. We have adopted the slope of the PL relation using time-averaged 3.6 μm data for 80 long-period Large Magellanic Cloud (LMC) Cepheids falling in the period range 0.8 < log(P) < 1.8. We find a new reddening-corrected distance to the LMC of 18.477 ± 0.033 (systematic) mag. We re-examine the systematic uncertainties in H0, also taking into account new data over the past decade. In combination with the new Spitzer calibration, the systematic uncertainty in H0 over that obtained by the Hubble Space TelescopeKey Project has decreased by over a factor of three. Applying the Spitzer calibration to the Key Project sample, we find a value of H0 = 74.3 with a systematic uncertainty of ±2.1 (systematic) km s–1 Mpc–1, corresponding to a 2.8% systematic uncertainty in the Hubble constant. This result, in combination with WMAP7 measurements of the cosmic microwave background anisotropies and assuming a flat universe, yields a value of the equation of state for dark energy, w0 = –1.09 ± 0.10. Alternatively, relaxing the constraints on flatness and the numbers of relativistic species, and combining our results with those of WMAP7, Type Ia supernovae and baryon acoustic oscillations yield w0 = –1.08 ± 0.10 and a value of Neff = 4.13 ± 0.67, mildly consistent with the existence of a fourth neutrino species. [ABSTRACT FROM AUTHOR]