Understanding the Universe: Origin, Expansion, and Cosmic Fate

Defining the Universe

The Universe is the totality of space and time, all forms of matter, energy, momentum, and the physical laws and constants that govern them. However, the term universe can be used in slightly different contextual senses, referring to concepts such as the cosmos, the world, or nature.

Age, Size, and the Big Bang

Astronomical measurements indicate that the Universe has an age of $13.73 \pm 0.12$ billion years and is at least 93 billion light-years wide.¹

The event believed to have begun the Universe is called the Big Bang. At that moment, all matter and energy in the observable Universe was concentrated at a point of infinite density. Following the Big Bang, the Universe began to expand to reach its current condition, and it continues to do so.

Cosmic Expansion and Relativity

According to the theory of Special Relativity, matter cannot move faster than the speed of light. It may seem paradoxical, therefore, that two objects in the Universe could have separated by 93 billion light-years in a time span of only 13 billion years.

However, this separation does not conflict with the theory of General Relativity, as Special Relativity only affects motion through space, not the expansion of space itself. Space can expand at a rate not limited by the speed of light. Therefore, two galaxies can separate from one another faster than the speed of light if the space between them is expanding.

Evidence Supporting Expansion

Key observations supporting the theory of expanding space and the Big Bang Theory include:

• Measurements of the spatial distribution and the redshift of distant galaxies.
• The existence of the Cosmic Microwave Background (CMB).
• The relative percentages of lighter chemical elements.

The Big Bang Theory posits that space itself was created from scratch at a specific time in the past.

Dark Matter, Dark Energy, and Acceleration

Recent observations have shown that this expansion is accelerating. Furthermore, most of the matter and energy in the Universe is fundamentally different from that observed on Earth and is not directly observable (see dark matter and dark energy). The imprecision of current observations has limited definitive predictions about the ultimate fate of the Universe.

Physical Laws and Dimensions

Experiments suggest that the Universe has been governed by the same physical laws and constants throughout its extent and history.

The dominant force across cosmic distances is gravity, which is most accurately described by the theory of General Relativity. The other three fundamental forces and the particles they operate on are described by the Standard Model.

The Universe has at least three dimensions of space and one of time. While extra dimensions too small to detect cannot be experimentally ruled out, space-time appears to be simply and smoothly connected. Space has a very small mean curvature, meaning that Euclidean geometry generally applies accurately across the cosmos.

Modeling the Cosmos

Philosophical and Scientific Perspectives

In philosophy, the Universe is often referred to as the world, or the set of everything that happens.

Science models the Universe as a closed system containing energy and matter bound to space-time, primarily governed by causal principles.

Based on observations of the observable Universe, physicists attempt to describe the continuous space-time we inhabit, along with all the matter and energy existing within it. The study of the Universe on the largest scales is the subject of Cosmology, a discipline based on astronomy and physics that describes all aspects and phenomena of the cosmos.

The Big Bang Model of Formation

The currently most accepted theory regarding the formation of the Universe, proposed by the Belgian priest and physicist Georges Lemaître, is the Big Bang Model. This model describes the expansion of space-time from a singularity.

The early Universe underwent a rapid period of cosmic inflation that smoothed out initial irregularities. Since then, the Universe has expanded, becoming stable, cooler, and less dense. Minor variations in mass distribution resulted in the segregation of matter into the structures found in the present Universe, such as clusters of galaxies.

The Ultimate Fate of the Universe

Regarding its ultimate fate, current evidence seems to support the theory of the permanent expansion of the Universe.

However, some theories suggest that dark matter could exert enough gravitational force to halt the expansion, causing all matter to compress back into a singularity—a scenario scientists call the Big Crunch.