Ask Ethan: Could The Shape Of Our Universe Be Closed Instead Of Flat?

Traditionally, it has been accepted that the Universe is flat, but recent findings indicate there may be more to the story.

The Universe is continually expanding and extends beyond the reach of our most advanced telescopes. Scientists have long believed that the observable portion of the Universe appears indistinguishably flat. However, a recent study utilizing data from the Planck satellite proposes a different viewpoint: the Universe might be curved with a closed geometry. This prompts inquiries, including one from Tom Ensalata, who wonders:

> "I thought the curvature parameter had been essentially settled by WMAP and Planck. What are your thoughts on the validity of this recent paper? Could the Universe be closed with a measurable positive curvature, as suggested by the authors?"

To address this, let's first delve into the concept of spatial curvature.

In flat space, as we typically encounter, the internal angles of any triangle will always total 180 degrees. This consistency holds true regardless of the triangle type—be it right, isosceles, or scalene—because they exist on a flat, two-dimensional plane.

However, if the two-dimensional surface is not flat, the triangle's angles can differ. For instance, if you modify a triangular piece of paper and reconnect it, the resulting angles can exceed 180 degrees, indicating positive curvature. Conversely, placing that triangle on a flat sheet can create a surface of negative curvature, leading to angles summing to less than 180 degrees.

The same principles apply to higher dimensions, including our entire Universe, which consists of three spatial dimensions and can exhibit positive, negative, or flat curvature. A positively curved Universe could resemble a sphere, yet still undergo expansion or contraction as it evolves.

The curvature of the Universe significantly influences its fate. If it consists solely of matter, the curvature determines whether the Universe is closed (and will eventually recollapse), critical (on the cusp of either fate), or open (destined to expand indefinitely).

The Universe's composition appears uniform across vast distances. This isotropy (uniformity in all directions) and homogeneity (uniformity in all locations) lead to significant equations in General Relativity, shaping our understanding of cosmic evolution.

Through these equations, we determine the Universe's scale changes over time and the various forms of matter and energy. If these elements misalign, the result is spatial curvature, leading to a flat Universe only when the expansion rate and overall energy density perfectly align.

The expanding Universe model has long suggested that while it may not be entirely flat, it is very close. A Universe that is significantly curved—positively or negatively—would either collapse quickly or expand too rapidly for galaxies and stars to form.

Our initial robust measurements supporting a nearly flat Universe came from the Cosmic Microwave Background (CMB). The BOOMERanG experiment in the late 1990s provided groundbreaking evidence that the Universe is very close to being spatially flat through direct measurements.

The CMB represents the residual glow from the Big Bang. While it seems uniform at a temperature of 2.725 K, slight fluctuations reveal density variations throughout the Universe. Where density is higher, light loses energy and appears colder; in underdense regions, light escapes more easily, appearing hotter.

This analysis of temperature fluctuations allows us to infer the Universe's geometry. The CMB measurements indicate a spatially flat Universe, with results refining our understanding over time—from BOOMERanG to WMAP, and now Planck.

Despite the compelling evidence, the CMB does not categorically prove a flat Universe, as the observed fluctuations can arise from multiple cosmological models. There remains flexibility within the CMB data, allowing for slight deviations toward closed or open models.

Recent research challenges the CMB conclusions, arguing that the lensing amplitude—an indicator of matter density—suggests a closed Universe. Gravitational lensing, which occurs when mass distorts light from distant sources, reveals greater-than-expected lensing signals, hinting at more matter density than previously thought.

Proponents of this new analysis suggest that a closed, overdense Universe could explain lower-than-expected temperature fluctuations at large scales. Additionally, adjusting curvature and energy content impacts the preferred value of the Hubble constant, which has been a point of contention in cosmology.

While the CMB suggests a Hubble constant of approximately 67 km/s/Mpc, other methods indicate a value closer to 73 km/s/Mpc. The new study finds that a slight overdense Universe aligns with data at a significant statistical level.

However, further exploration of this hypothesis reveals complications. Adjusting the cosmological model to account for more matter and a closed Universe exacerbates the tension regarding the Hubble constant, pushing values into the mid-50s, which are problematic.

Moreover, the most robust constraints on spatial curvature now derive from baryon acoustic oscillations (BAO). By examining the large-scale structure of the Universe, we find it is spatially flat to within 0.4% precision, a more stringent constraint than provided by the CMB.

While exploring alternative cosmological explanations is an essential part of scientific inquiry, any new models must align with comprehensive evidence. The notion of a closed, overdense Universe, while intriguing, ultimately faces significant challenges and inconsistencies with existing data.

> Send in your Ask Ethan questions to startswithabang at gmail dot com!

> Starts With A Bang is now on Forbes, and republished on Medium thanks to our Patreon supporters. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.