Chapter 1: The Cosmic Expansion Conundrum

Recent studies have shed light on the ongoing debate surrounding the Hubble Constant — a crucial measurement that indicates the rate at which the universe expands. New findings using Red Giant stars suggest that this rate might lie between previously conflicting estimates.

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Section 1.1: Conflicting Estimates of Cosmic Expansion

The determination of the universe’s expansion rate has emerged as one of the most contentious issues in modern astrophysics. Current methodologies for calculating the Hubble Constant are yielding differing results, creating a significant challenge for scientists.

The galaxies featured in this study were selected from a Hubble Space Telescope initiative aimed at measuring the Hubble Constant. This value is derived by analyzing the distances of these galaxies in relation to their observed rate of receding from Earth, a phenomenon influenced by the expanding universe. By comparing the apparent brightness of red giant stars in these galaxies to those nearby, astronomers can accurately gauge their distances. Red giants serve as reliable indicators, as they consistently achieve the same peak brightness during their later life stages, acting as “standard candles” for distance calculations. The Hubble Space Telescope's high-resolution capabilities allowed researchers to pinpoint these red giants within the galaxies' stellar halos.

Now, a team from Carnegie and the University of Chicago has introduced a new approach to measuring the expansion rate, utilizing red giants instead of traditional methods.

Chapter 2: Three Approaches to Measure the Hubble Constant

The first traditional method involves Cepheid variables — stars that pulsate at fixed intervals. Their luminosity directly relates to their pulsation period, allowing astronomers to accurately calculate their distances from Earth.

Carnegie's Barry Madore, a co-author of the study, notes: “By comparing the apparent brightness of distant Cepheids with those nearby, we can determine the distance to their host galaxies.”

Once a celestial object's distance is established, astronomers can assess its velocity relative to Earth by measuring its redshift, thereby estimating the Hubble Constant.

The second method employs the Planck model, which analyzes the cosmic microwave background (CMB) radiation leftover from the Big Bang. This remnant light, remarkably uniform across the sky, allows scientists to infer the universe's early conditions and predict the current Hubble Constant.

Currently, these two methods yield conflicting results: one suggests an expansion rate of 74.0 kilometers per second per megaparsec, while the other indicates 67.4 kilometers per second per megaparsec. Such a significant difference could imply the need for new physics to explain these observations.

The first video, The Cosmic Expansion Hypothesis is Falsified, the Universe is not Expanding, discusses the implications of these divergent measurements and challenges conventional understandings of cosmic expansion.

The Carnegie-Chicago Hubble Program, led by Freedman and her colleagues, has developed a fresh technique centered on luminous red giants. When these stars ignite helium in their cores, they undergo structural changes that produce a distinct peak brightness, making them excellent standard candles.

Madore elaborates: “Just as a loon’s cry is instantly recognizable, the peak brightness of a red giant during this phase is easily identifiable. This characteristic allows us to calculate distances with precision.”

Using the Hubble Space Telescope's advanced imaging capabilities, the team searched for red giants in nearby galaxies. Burns compares this effort to identifying the tallest person in a crowd: “If you know the tallest person in any room has a specific height, you can use that information to estimate their distance from you.”

The second video, Do we live in a giant void that could solve the puzzle of the universe's expansion?, explores how our understanding of cosmic structures might affect measurements of the Hubble Constant.

Once the distances to these red giants are established, researchers can utilize another type of standard candle — type Ia supernovae — to further refine the Hubble Constant calculation, extending the reach into the more distant Hubble flow.

According to the red giant method, the calculated expansion rate of the universe is 69.8 kilometers per second per megaparsec, intriguingly positioned between the previously established values.

While these results do not decisively favor one method over another, they align more closely with the Planck model findings.

NASA's forthcoming Wide-Field Infrared Survey Telescope (WFIRST), set to launch in the mid-2020s, aims to enhance our understanding of the Hubble Constant across cosmic epochs. With its Hubble-like resolution and an expansive field of view, WFIRST will provide a wealth of data on type Ia supernovae, Cepheid variables, and red giants, significantly improving distance measurements to galaxies near and far.

Madore humorously remarks: “We feel like the old song, ‘Stuck in the Middle with You.’ Is there a crisis in cosmology? We had hoped to be the tiebreaker, but for now, the answer is: not so fast. The question of whether the standard model of the universe is truly complete remains open.”