What If The Universe DID NOT Start With The Big Bang?

Summary

This PBS Space Time episode explores the concept of the universe's beginning, questioning if it started from a singularity. It discusses the Big Bang, cosmic inflation, and the possibility of an eternally inflating universe. The script delves into geodesic incompleteness and the Borde-Guth-Vilenkin theorem, suggesting the universe likely had a beginning. However, it also presents a scenario where our universe might be part of a larger, extendable spacetime, challenging the notion of a definitive start.

Takeaways

• 🌌 The universe is expanding, but if you rewind time, all of space was compacted into an infinitesimal point—the Big Bang singularity.
• 🧐 Early models based on Einstein's general relativity suggested a past singularity, but assumptions like perfect smoothness were made.
• 🌠 Cosmic inflation helped explain the smoothness of the universe, but this period may not have ended everywhere, leading to the idea of eternal inflation.
• 🔄 Recent theories explore whether eternal inflation could mean the universe had no true beginning, challenging the idea of a past boundary.
• 🚫 Geodesic incompleteness suggests the universe likely had a beginning since all spacetime paths converge at a single point in the past.
• 🕳️ Just like black holes have a singularity, the universe might have one too, though the event horizon can be crossed via coordinate transformation.
• 🔍 The Borde-Guth-Vilenkin theorem states that any universe with an average expansion rate greater than zero must have a beginning.
• 📉 Some expansion histories might allow universes to avoid singularities, but these require breaking some rules of general relativity.
• 🌀 The transition from our universe's FLRW spacetime to larger de Sitter space might suggest the past boundary is just a coordinate singularity.
• 🧪 The lumpy structure of the early universe, required for galaxy formation, suggests a real singularity and a possible beginning to spacetime.

Q & A

What is the singularity at the beginning of the universe?

The singularity at the beginning of the universe is a point of infinite density and infinitesimal size, where all of space was compacted at the start of time, according to the Big Bang theory.

How does the concept of cosmic inflation relate to the beginning of the universe?

Cosmic inflation is a theory that suggests the universe underwent a rapid exponential expansion in a tiny fraction of a second after the Big Bang, which helps explain the observed uniformity of the universe on a large scale.

What is a past singularity and why is it significant?

A past singularity refers to a point in time where all points in space converge, suggesting a beginning of time. Its existence or non-existence is key to understanding whether the universe had a definite start.

What assumptions did early models of the expanding universe make that could affect our understanding of the Big Bang?

Early models, like those by Alexander Friedmann and Georges Lemaitre, assumed the universe was perfectly smooth everywhere, which is not the case as evidenced by the existence of galaxies, potentially affecting the conclusion of a singular starting point.

What is meant by 'eternal inflation' and how does it challenge the idea of a beginning of the universe?

'Eternal inflation' refers to the idea that inflationary periods of the universe might continue indefinitely into the future and, if so, could also have extended indefinitely into the past, implying the universe may have had no beginning.

How does the concept of geodesic incompleteness relate to the potential beginning of the universe?

Geodesic incompleteness refers to the inability to trace a geodesic (the shortest path through spacetime) indefinitely into the past, suggesting a possible end or beginning of spacetime.

What is the Borde–Guth–Vilenkin (BGV) theorem and what does it imply about the universe's beginning?

The BGV theorem posits that any universe that has been expanding on average throughout its history cannot have been expanding forever and must have a past boundary, suggesting a beginning.

How do curvature singularities differ from coordinate singularities, and why is this distinction important?

Curvature singularities imply a physical end of spacetime due to infinite gravitational warping, whereas coordinate singularities are points where a particular coordinate system fails, but spacetime may still extend beyond them. Understanding this helps determine if the universe's beginning is a physical reality or a mathematical artifact.

What role does the concept of de Sitter space play in the discussion of the universe's possible extension beyond its past boundary?

De Sitter space is a model of spacetime that includes exponential expansion and can be considered a larger context for our universe. The possibility that our universe is a segment of a larger de Sitter space challenges the BGV theorem's assertion of a past boundary as a definite beginning.

How do density fluctuations early in the universe's history impact the likelihood of a singularity at its beginning?

Significant density fluctuations early in the universe's history could prevent spacetime from being extendable past the past boundary, suggesting a physical singularity and a definite beginning of time.

What is the significance of the Penrose diagram in understanding the universe's beginning?

A Penrose diagram is a graphical tool used to visualize spacetime by combining space and time into a two-dimensional representation. It can reveal potential extensions of spacetime beyond apparent boundaries, such as past singularities.