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The evolution of the "microscopic" Hubble parameter related to the expansion of matter born in heavy-ion collisions was obtained for nucleons and pions. The calculations were carried out within the parton-hadron-string dynamics (PHSD) transport model. Au+Au collisions with $\sqrt{s_{NN}} = 7.8$ GeV at $b = 2.5,\ 5.0,\ 7.5$, and $10.0$ fm were considered. A new method for determining the "microscopic" Hubble parameter from simulated data was used. The ballistic motion was obtained for the longitudinal direction after the separation of the nuclei. In earlier times, the evolution of the "microscopic" Hubble parameter in this direction was more complicated. For transverse directions, an exponential low-time asymptotics of the Hubble parameter was observed. The obtained values of the "microscopic" Hubble parameter are about 40 orders of magnitude higher than the cosmological Hubble constant.

Ever since the new millennium, precision cosmology has forged the $Λ$-cold-dark-matter ($Λ$CDM) model as the standard model of concordant cosmology, withstanding various tests except for an ever-enlarging discrepancy between early-Universe observations and late-Universe measurements on the current Hubble expansion rate of our observable Universe. This Hubble-constant tension has likely become a real crisis for modern cosmology, with the discrepancy persisting regardless of whether the early-Universe observations depend on \textit{Planck} CMB or not, and the late-Universe measurements depend on distance ladders at all. If the Hubble tension originates from a different early Universe, its resolutions pertain to shrinking the sound horizon by altering either early expansion or recombination histories, but at the same time necessitating modifications to both primordial and late Universe altogether. Alternatively, if the Hubble tension arises from a different late Universe, its resolutions operate by changing the absolute magnitude of supernovae either intrinsically or effectively, both of which have been strongly constrained by the inverse distance ladders with the cosmic distance dual

The Hubble Space Telescope inaugurated the era of exoplanet atmospheric characterization. While the James Webb Space Telescope has largely taken up the mantle of infrared atmospheric characterization, Hubble's unique short-wavelength capabilities remain unmatched. Recent theoretical advances in exoplanet atmospheric science combined with new observing strategies, like those offered by WFC3-UVIS/G280, have opened science cases that only Hubble can address for the foreseeable future. In this white paper, we discuss these new windows into the atmospheres of other worlds, focusing on characterization of their hydrostatic lower atmosphere, and identify the critical capabilities necessary for future observations. We highlight three overall science cases that will depend on the continued short-wavelength capabilities of Hubble: measuring aerosol scattering slopes, characterizing metal absorption in ultra-hot Jupiters, and understanding stellar activity with Transit Light Source effect decontamination and flare monitoring. Throughout, we highlight useful synergies between HST and JWST. This article is a response to the call for white papers by the Space Telescope Science Institute on "Buil

Hubble's long, stable astrometric baseline creates a rare opportunity for discovery in the Local Group and beyond. Many nearby galaxies, streams, and star clusters already have archival first-epoch imaging in hand, so future HST observations over the next decade can turn those data into precise proper motions. For many Milky Way satellites, existing measurements already constrain orbital motion at a useful level, but HST still offers a path to full 3D kinematics, internal motions, and more distant systems where current data remain insufficient. That opens the window to dynamical studies inaccessible through line-of-sight velocities alone, revealing orbital histories, internal kinematics, environmental processing, and the dark-matter structure of nearby galaxies. This white paper identifies HST astrometry as an opportunity to capitalize on archival baselines by completing long-baseline measurements where first epochs already exist, establishing new first epochs where critical gaps remain, and assembling a legacy sample for future JWST, Roman, and HWO-era follow-up. The result will be a transformative dataset for the Local Group and Local Volume, driving discovery now while laying th

An important evolutionary pathway for planetary atmospheres is escape to space, which has been studied on Earth and Mars for several decades and more recently in exoplanets. A particularly important regime is the hydrodynamic escape, wherein atmospheric mass escapes the planet at high rates in a collisional fluid outflow. This process is used to partly explain the early evolution of rocky planets in and out of the Solar System, as well as key aspects of exoplanet demographics. Hydrodynamic escape is not occurring in the Solar System planets, so our only option for such observations is through exoplanets. The ultraviolet (UV) capabilities of the Hubble Space Telescope (HST) are fundamental to detect hydrodynamic escape and measure the resulting mass-loss rates for a range of planetary systems and to identify targets for surveys with the Habitable Worlds Observatory. We discuss here what kinds of observations and instrument modes are necessary to continue studying atmospheric escape in exoplanets for the next decade, as well as how to advance our understanding of planetary evolution and habitability.

The spatial distribution and lifetime of molecular gas in the inner regions of young circumstellar disks are key to understanding the formation of planetary systems. Gas-rich disks are observed to disperse in the first ~10 Myr, and recent observational and theoretical evidence suggests that circumstellar disks winds may dominate the removal of angular momentum from the disk, allowing it to dissipate through accretion onto the central star and through low-velocity (<~30 km/s) outflows. The Hubble Space Telescope has revolutionized our understanding of the disks, accretion, and outflow processes that drive the evolution of planet-forming disks and is poised to answer the key questions in the field in the coming decade. We describe how HST's ultraviolet capabilities can address these questions and identify key goals and high-priority observations for HST into the 2030s.

The most irreplaceable capability of HST in the 2030s is not only its angular resolution or its UV--optical sensitivity, but its accumulated time baseline. We recommend a Hubble Local Group Astrometric Legacy that would obtain matched 2030s ACS/WFC and WFC3/UVIS imaging of selected M31 and M33 star cluster fields, combine those observations with a careful astrometric audit of the existing archive, and deliver public transverse velocity products through MAST. PHAT and PHATTER already provide exceptional first epochs across the nearest large external spirals, resolving tens to hundreds of millions of stars and identifying thousands of star clusters and compact background reference objects. Earlier targeted HST programs, dating back to the mid 90's, can extend the temporal baseline in selected cases, but only after camera, chip, filter, dithering, crowding, and reference-frame triage. A coherent 2030s repeat campaign would turn the best of this archive into bulk cluster motions, enabling the first systematic orbital mapping of star cluster populations beyond the Milky Way. These measurements would constrain disk heating, cluster disruption, accretion histories, stream associations, th

The Hubble tension is commonly framed as a discrepancy between local, late-time measurements favoring $H_0 \approx 73$ km s$^{-1}$ Mpc$^{-1}$ and early-time, Sound-Horizon-based measurements favoring $H_0 \approx 67$ km s$^{-1}$ Mpc$^{-1}$. We challenge this viewpoint by analyzing 88 Sound Horizon Free $H_0$ measurements, categorized into four classes: Distance Ladder measurements using local calibrators; Local $Λ$CDM measurements assuming the standard expansion history; Pure Local measurements independent of $H(z)$ shape; and CMB Sound--Horizon--Free measurements using CMB data without the Sound Horizon scale. Our analysis reveals that the 30 Distance Ladder measurements yield $H_0 = 72.73 \pm 0.39$ km s$^{-1}$ Mpc$^{-1}$ ($χ^2_ν= 0.72$), while the 58 Distance Ladder-Independent/Sound Horizon Free measurements collectively yield $H_0 = 69.37 \pm 0.34$ km s$^{-1}$ Mpc$^{-1}$ ($χ^2_ν= 0.95$), a $6.5σ$ tension exceeding the Planck--SH0ES discrepancy. This tension remains significant at a minimum value of $3.9σ$ after accounting for correlations. Among categories, Local $Λ$CDM measurements favor the lowest value ($H_0 = 67.61\pm 0.96$ km s$^{-1}$ Mpc$^{-1}$), Pure Local yield an inter

The Habitable Worlds Observatory (HWO) will provide the first opportunity to directly image and spectrally characterize terrestrial exoplanets in the habitable zones of nearby stars. Maximizing its scientific return requires a comprehensive understanding of the high-energy radiation environments of target stars, which shape planetary atmospheres and govern the production, destruction, and detectability of biosignatures. Ultraviolet (UV) radiation plays a particularly critical role in atmospheric chemistry. Far-ultraviolet (FUV) and near-ultraviolet (NUV) photons regulate key photochemical pathways, influence ozone stability, and drive the formation of prebiotic molecules. However, the majority of high-priority HWO target stars lack high-quality UV observations. Existing datasets are sparse, heterogeneous, or limited by calibration uncertainties, and no comparable UV observatory is expected for at least 5-10 years (with UVEX offering more limited spectral resolution, wavelength coverage, and sensitivity). The Hubble Space Telescope (HST) remains the only observatory capable of acquiring high-resolution FUV and NUV spectra for these targets over the next 10-15 years. We therefore adv

The chemical and mass evolution of exoplanet atmospheres is shaped by their specific X-ray through ultraviolet (5 - 3200 Angstroms) irradiance history. X-ray and EUV (5 - 911 Angstroms) radiation largely contributes to atmospheric heating via photoionization, while far- and near-UV emission (912 - 3200 Angstroms) drives photochemistry. The (uncharacterized) variance between young star spectra in this wavelength range for the same spectral type causes significant uncertainty in interpreting present-day transmission spectra of young exoplanets, directly impacting the science return of the James Webb Space Telescope and other programs. Additionally, the lack of direct X-ray through UV characterization for stars of all ages leads to large uncertainties in the high-energy irradiance history of all planetary systems, propagating into uncertainties in their chemical and mass evolution. This influences current and future observing programs, as well as the goal of the future flagship Habitable Worlds Observatory to find and characterize habitable exoplanets. There are less than a handful of young planet hosts with well-characterized X-ray through UV environments. The Hubble Space Telescope

The Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope currently stands as the sole space-based astronomical facility providing visible-light coronagraphic imaging -- and the only facility anywhere that can perform both visible- and ultraviolet-light coronagraphic spectroscopy. In imaging, STIS offers unparalleled stability that rivals the performance of ground-based direct imaging in the optical, and a wide field of view that will complement the upcoming capabilities of Roman coronagraphy. STIS also has the capability for direct high-contrast visible and ultraviolet spectroscopy via two occulting bars in its 52''$\times$0.2'' spectroscopic slit. By placing a bright astrophysical source behind an occulting bar, it is possible to use the STIS visible and NUV/FUV gratings to obtain spatially-resolved spectra of faint environments and companions, covering wavelengths from 1150-10,300Å at resolutions of $R\sim500-10{,}000$. In this white paper, we detail the use cases and performance of this under-utilized mode, with starlight subtraction enabling visible light spectral contrasts of $\sim10^{-4}-10^{-5}$. We describe the promise of STIS coronagraphic spectroscopy

Boosting the Hubble Space Telescope (HST) will provide unique opportunities to carry out precursor science for the Habitable Worlds Observatory (HWO). Chief among them are science cases for determining the properties of star forming galaxies that contribute to creating and sustaining the universe in a mostly ionized state. The farUV and nearUV spectroscopic capabilities of the Cosmic Origins Spectrograph (COS) and the Space Telescope Imaging Spectrograph (STIS) are unique and unlikely to be replicated in the near future. Here we describe the benefits of a deep panchromatic spectroscopic effort to answer questions concerning the shape of the rest frame ionizing radiation escaping from star forming galaxies at modest redshift, capture crucial missing spectral regions, and explore whether their star formation histories are truly similar to the LyC leakers responsible for initiating and later sustaining the mostly-ionized-state of the universe. An observing program emphasizing multi-orbit observations, unencumbered by HST orbit competition and freely accessible to the wider ionizing photon (iPhoton) community, will catalyze crowd-sourced answers to these questions and offer a lower ope

While the tip of the red giant branch (TRGB) has been used as a distance indicator since the early 1990's, its application to measure the Hubble Constant as a primary distance indicator occurred only recently. The TRGB is also currently at an interesting crossroads as results from the James Webb Space Telescope (JWST) are beginning to emerge. In this chapter, we provide a review of the TRGB as it is used to measure the Hubble constant. First, we provide an essential review of the physical and observational basis of the TRGB as well as providing a summary for its use for measuring the Hubble Constant. More attention is then given is then given to recent, but still pre-JWST, developments, including new calibrations and developments with algorithms. We also address challenges that arise while measuring a TRGB-based Hubble Constant. We close by looking forward to the exciting prospects from telescopes such as JWST and Gaia.

Current observations with the James Webb Space Telescope (JWST) suggest that star-forming galaxies produce enough ionizing (LyC; $λ< 91.2$ nm) photons to drive cosmic reionization, but the efficiency with which these photons escape their host galaxies remains uncertain. Absorption by the neutral intergalactic medium progressively suppresses direct LyC detections above redshift $z\sim3$, forcing astronomers to rely on indirect diagnostics of LyC escape calibrated at low redshift. Low-resolution ultraviolet observations of high-redshift analogs obtained with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope (HST) have been critical for developing these diagnostics. These studies suggest that stellar feedback plays a central role in regulating LyC escape, although the role of galactic winds and the underlying physical mechanisms remain poorly constrained. High-resolution spectroscopy blueward of 160.0 nm (rest-frame) is required to resolve the kinematic structure of the winds and reveal the physics governing LyC escape. Such observations are currently only possible with HST and represent a major science driver for the future Habitable Worlds Observatory (HWO). Exte

We test whether $f(Q)$ symmetric teleparallel gravity theories can solve the Hubble tension consistently with DESI DR2 BAO. We consider three $f(Q)$ functional forms: logarithmic, exponential, and hyperbolic tangent. We extend these models by allowing a cosmological constant, and compare to phenomenological models with a flexible exponential, hyperbolic secant, and polynomial decay addition to the standard $Λ$CDM $H(z)$. We test these models against DESI DR2 BAO, CMB ($Planck$ 2018 + SPT-3G + ACT DR6), local $H_0$, and Cosmic Chronometer data. The logarithmic and hyperbolic tangent $f(Q)$ models do not provide an adequate solution, but the exponential model does. Furthermore, it slightly reduces the $(Ω_m, H_0 r_d)$ parameter space tension between CMB and BAO datasets to $2.56σ$, down from $2.65σ$ for $Λ$CDM. Although $Λ$CDM faces only $1.66σ$ tension in DESI data space, the $1σ$ higher tension in parameter space suggests a real anomaly. The models assisted by the cosmological constant perform slightly better still, at the cost of undermined theoretical motivation. They also perform poorly once local $H_0$ measurements are included. The phenomenological models fit all data reasonab

We present a new Hubble parameterization method and employ observational data from Hubble, Pantheon, and Baryon Acoustic Oscillations to constrain model parameters. The proposed method is thoroughly validated against these datasets, demonstrating a robust fit to the observational data. The obtained best-fit values are $H_0 = 67.5^{+1.3}_{-1.6}$ $\text{km s}^{-1} \text{Mpc}^{-1}$, $Ω_{\rm{m0}} = 0.2764\pm{0.0094}$, and $α= 0.33\pm{0.22}$, consistent with the Planck 2018 results, highlighting the existence of Hubble tension.

The Hubble constant ($H_0$), which represents the expansion rate of the Universe, is one of the most important cosmological parameters. The recent measurements of $H_0$ using the distance ladder methods such as Type Ia Supernovae (SNe Ia) are significantly greater than the CMB measurements by Planck. The difference points to a crisis in the standard model of cosmology termed as Hubble tension. In this work we compare different cosmological models, determine the Hubble constant and comment on the Hubble tension using the data from differential ages of galaxies. The data we use is free from the systematic effects as the absolute age estimation of the galaxies is not needed. We have used the Bayesian approach along with the commonly used maximum likelihood method to estimate $H_0$ and have calculated the AIC scores to compare the different cosmological models.The non-flat cosmological model provides a higher value for matter density as well as the Hubble constant compared to the flat $Λ$CDM model. The AIC score is smaller for the flat $Λ$CDM cosmology compared to the non-flat model indicating the flat model a better choice. The best-fit value of $H_0$ for both these models are $68.7\p

Recently Block published an astro-ph{http://arxiv.org/abs/1106.3928 (2011).} insinuating that Lemaitre discovery paper of the Expanding Universe was censored prior to its translation into English and publication in the Monthly Notices of the Royal Astronomical Society. Consequently, Lemaitre's credit for the discovery of the velocity-distance correlation was not recognized. We examine here the chain of events leading to the discovery of the 'Hubble law'. Our summary: (a) Lemaitre found a theoretical linear correlation between velocity and distance. (b) Lemaitre assumed the existence of a linear relation between velocity and distance and calculated the coefficient. (c) Hubble took the data plotted it and demonstrated that a linear relation represents the observed data. (d) Hubble never believed in Lemaitre's solution, namely in an expanding universe. Consequently, Hubble never cited Lemaitre. We conclude that the charge that Lemaitre's paper was censored or ignored let alone plagiarized by Hubble, is not founded, and explain why Lemaitre's earlier theoretical discovery and derived 'Hubble constant' was not cited or recognized, by Hubble as well as by many other leading researchers.

Dark radiation, parameterized in terms of $N_{\rm eff}$, has been considered many times in the literature as a possible remedy in alleviating the Hubble constant ($H_0$) tension. We review here the effect of such an extra dark radiation component in the different cosmological observables, focusing mostly on $H_0$. While a larger value of $N_{\rm eff}$ automatically implies a larger value of the Hubble constant, and one would naively expect that such a simple scenario provides a decent solution, more elaborated models are required. Light sterile neutrinos or neutrino asymmetries are among the first-order corrections to the most economical (tree-level) massless dark radiation scenario. However, they are not fully satisfactory in solving the $H_0$ issue. We devote here special attention to second-order corrections: some interacting scenarios, such as those with new dark radiation degrees of freedom that exhibit a non-free streaming nature are highly satisfactory alternative cosmologies where to solve the Hubble constant tension. Models with self-interacting sterile neutrinos and/or majorons, both well-motivated beyond the Standard Model particles, will be discussed along our assessmen