Theoretical Insights and Observational Challenges

Long before dark energy was confirmed through observation, esteemed physicist Steven Weinberg grappled with the theoretical overestimations of vacuum energy. He proposed an intriguing explanation: the vacuum energy can fluctuate based on the interplay of various fields throughout the universe. In our particular region, the vacuum energy is remarkably low, implying a near-perfect cancellation among these fields. While this might seem improbable, in sufficiently large systems, even the most unexpected events can occur. Most cosmic regions experience an excess of dark energy, disrupting the balance of forces necessary for the formation of galaxies, stars, and planets. This suggests that the precise cancellation observed in our vicinity is a necessity for our existence.

Recent computer simulations have been testing Weinberg’s hypothesis, revealing that galaxies exhibit resilience beyond previous assumptions. Their formation appears to be more influenced by internal dynamics than by the external pull of dark energy. If this is indeed the case, then the amount of dark energy we observe cannot be merely a selection effect, leaving us to reassess our comprehension of dark energy.

The complex interplay of cosmic forces—gravity, pressure, cooling, heating, expansion, and collapse—shapes our environment. Ultimately, this delicate balance results in the Milky Way galaxy, where stars are birthed in a rotating disk of gas accompanied by a diffuse halo of dark matter.

In the first video, "HEP Seminar - How to Destroy a Galaxy with Dark Matter," we explore the mechanisms by which dark matter can influence galaxy formation and stability.

Formation Processes of Galaxies

To comprehend the intricate processes of galaxy and star formation, simulations segment matter into smaller units affected by gravity, pressure, and other forces. These fragments exist within an expanding universe, where their gravitational influences on one another are calculated. The initial balance to evaluate is between gravity and expansion—the former draws matter together while the latter pushes it apart. In sufficiently dense regions, gravity prevails, attracting additional matter from surrounding areas. As a result, our initially homogeneous universe begins to exhibit structure.

Matter consolidates into somewhat spherical “haloes,” which then reach a new equilibrium, balancing gravitational forces against the chaotic motion of matter. To facilitate galaxy formation, it is crucial for gas to cool down. Initially hot gas in these haloes emits thermal energy, disrupting the balance. Eventually, gravity triumphs, leading to matter collapsing into a stable configuration, supported by rotational motion. This process births a galaxy, enabling star formation within its disk.

Interestingly, simulations that incorporate the discussed physics have proven too effective at creating galaxies and stars, outperforming actual observations. The missing element in these simulations is feedback: as stars form, they release energy back into their surroundings, heating the gas. When massive stars exhaust their fuel, they explode, ejecting matter from galaxies. This feedback mechanism actually inhibits star formation, allowing the observable balance to materialize.

These simulations showcase a detailed web of cosmic structure, merging dark matter and ordinary matter to create strikingly realistic galaxies. If Weinberg’s theory holds true, this process should falter in regions characterized by higher dark energy.

The second video, "Physicists Reacts to The Black Hole That Kills Galaxies," provides insights into how black holes can affect galactic formation and the implications of dark energy.

Galactic Isolation and Implications of Dark Energy

To illustrate the potential effects of varying dark energy levels, let’s follow the matter that could become a galaxy like our Milky Way. In our simulated universe, this matter initially fragments into smaller components, some of which evolve into small galaxies. The young Milky Way expands by accumulating gas to form stars and assimilating smaller galaxies. Today, we observe long, thin streams of stars around the Milky Way—remnants of those smaller, once-independent galaxies.

As we increase the dark energy levels in our model, the universe's accelerating expansion phase begins earlier. Consequently, the competition between gravity and cosmic expansion shifts in favor of expansion sooner, ultimately stalling the formation of structures. Smaller galaxies that would have merged with the young Milky Way are swept away by the expanding universe. The gas that could have fueled star formation never comes close enough to the galaxy’s gravitational influence. Thus, the young Milky Way becomes isolated and lacks the resources to thrive, leading to a stagnant state.

The Fate of Cosmic Structure

As we further escalate dark energy levels, structure formation ceases even before the smallest galaxies can materialize. The universe becomes a barren expanse, with no galaxies, stars, planets, or life. It remains a diffuse gas of particles that rarely interact.

At first glance, our findings lend credence to Weinberg’s hypothesis, yet there is an unexpected twist. In our universe, star formation was at its peak several billion years before the onset of dark energy-driven expansion. By the time galaxies became isolated due to cosmic acceleration, star production was already on the decline. Stellar feedback significantly impacts the efficiency of star formation, relegating dark energy to a secondary role in this dynamic.

Consequently, even a tenfold increase in dark energy yields minimal impact. It is only with a substantial increase—still under investigation—that cosmic acceleration truly stifles star formation.

Our research is ongoing, and we continue to analyze data. However, initial indications suggest that alternative explanations for dark energy may warrant further exploration.

Originally published at Nautilus on March 23, 2017.