Dark Energy, Cosmology part 2: Crash Course Astronomy #43

Summary

This episode of Crash Course Astronomy explores the surprising discovery that the Universe's expansion is accelerating, contrary to expectations. It delves into the concept of dark energy, which is thought to be responsible for this acceleration, and discusses the implications for the Universe's future, including the shrinking of the observable Universe.

Takeaways

• 🌌 The Universe is expanding, with galaxies moving away from each other due to the expansion of space itself.
• 🔴 The Big Bang is the current working model for the start of the Universe, occurring about 13.82 billion years ago when all matter and energy were compressed into a singularity.
• 🌐 On large scales, the expansion of the Universe is faster, causing distant galaxies to appear as if they are rushing away from us.
• 🌠 The expansion of the Universe is influenced by gravity, but on small scales, gravity can overcome the expansion, as seen with the Andromeda galaxy moving towards us.
• 💥 The discovery of dark matter was expected to slow down the expansion of the Universe, but instead, observations showed an acceleration.
• 🌌 Astronomers used Type Ia supernovae as standard candles to measure the expansion rate of the Universe, finding that it is accelerating rather than slowing down.
• 🚀 The acceleration of the Universe's expansion is attributed to dark energy, an invisible form of energy that permeates space and pushes on the expansion.
• 🌟 Dark energy is estimated to make up about 68% of the total energy content of the Universe, significantly more than normal matter.
• 🌍 The geometry of the Universe, which describes its overall curvature, was once thought to determine its fate, but the presence of dark energy has complicated this.
• 🌠 The observable Universe has a radius of about 45 billion light years, but due to the accelerating expansion, this cosmic horizon is effectively moving closer, making the Universe appear to shrink in our view.

Q & A

What is the singularity in the context of the Big Bang theory?

The singularity refers to the point in time, 13.82 billion years ago, when all of space, time, matter, and energy were compressed into an infinitely dense point, initiating the Big Bang and the expansion of the Universe.

Why do distant galaxies appear to be moving away from us faster?

Distant galaxies appear to be moving away faster due to the expansion of space itself; the larger the scale, the faster the expansion, which is why their recession velocities are higher.

How does the Andromeda galaxy's motion counteract the expansion of space?

The Andromeda galaxy is moving toward us due to the mutual gravitational pull between our galaxies, which is strong enough to overcome the expansion of space on a local scale.

What is the significance of Type Ia supernovae in measuring cosmic distances?

Type Ia supernovae are significant because they have a consistent peak luminosity, making them 'standard candles'. By comparing their known intrinsic brightness with their observed brightness, astronomers can calculate their distances.

Why was the discovery that the Universe's expansion is accelerating so surprising?

It was surprising because astronomers expected gravity from all the matter in the Universe to slow down the expansion. Instead, they found evidence of an unknown force, later termed 'dark energy', that is causing the expansion to accelerate.

What is dark energy and how does it affect the Universe's expansion?

Dark energy is a mysterious form of energy that permeates all of space, exerting a negative pressure that causes the expansion of the Universe to accelerate, contrary to the deceleration expected due to gravity.

How does the concept of 'escape velocity' relate to the expansion of the Universe?

Escape velocity is analogous to the Universe's expansion in that if an object (or galaxy) is moving fast enough, it can escape the gravitational pull (or expansion) and move away indefinitely. If not, it slows down and falls back.

What is the current understanding of the Universe's ultimate fate given the presence of dark energy?

The current understanding, supported by measurements of dark energy, suggests that the Universe will continue to expand forever, with the expansion rate accelerating over time.

What is the observable Universe and how is its size affected by the expansion?

The observable Universe is the part of the Universe from which light has had time to reach us, currently about 45 billion light years away. Its size is affected by the expansion of space, which, due to dark energy, is accelerating, making the observable Universe effectively shrink over time.

Why is the cosmic horizon approaching us and what are its implications?

The cosmic horizon is approaching due to the accelerating expansion of space caused by dark energy. This implies that in the future, we will only be able to see galaxies within our local group, as all others will recede beyond the cosmic horizon, making the night sky darker.

How does the concept of the Universe's geometry relate to its fate?

The geometry of the Universe, which describes its overall curvature and the shape of space on the largest scales, was once thought to determine whether the Universe would expand forever or eventually recollapse. However, the discovery of dark energy has shown that the Universe will likely expand indefinitely, regardless of its geometry.