Ep. 7: What’s Keeping the Stars Apart | Crash Course Pods: The Universe #7

Introduction (0s)

• The episode discusses cosmic noon, a period involving the creation of most of the stuff in the Milky Way and the universe.
• Cosmic noon is significant because it marks a point near the present in the history of the universe and involves the mystery of an accelerated cosmic expansion.
• The episode explores cosmologists' attempts to answer big questions about our origins on a grand scale.
• The discussion begins by establishing the timeline and the concept of time.
• Cosmic noon refers to a period in the history of the universe when star formation peaked.
• It occurred approximately 10 billion years ago.
• During cosmic noon, the universe was filled with bright, young stars, and the cosmic microwave background radiation was at its peak.
• The accelerated expansion of the universe is a mystery that cosmologists are still trying to understand.
• One theory suggests that a "secret cosmic wind" or "dark energy" is responsible for this expansion.
• Dark energy is a hypothetical form of energy that permeates the universe and causes it to expand at an accelerating rate.
• The nature of dark energy is still unknown, and it is one of the biggest mysteries in cosmology.
• The episode discusses a theory for how our solar system came to be.
• According to this theory, the solar system formed from a giant molecular cloud that collapsed under its own gravity.
• The cloud fragmented into smaller clumps, one of which eventually became the sun.
• The remaining clumps of gas and dust formed the planets, moons, and other objects in the solar system.

Reviewing our Timeline (1m25s)

• The discussion deviated from the timeline to cover dark matter, black holes, and the Epic of Reionization.
• The Epic of Reionization marks the point when stars emit enough light to ionize the intergalactic medium, allowing the universe to become transparent to visible light.
• The universe is currently in a state where most of the intergalactic medium is ionized gas, allowing starlight to pass through unimpeded.
• The formation of galaxies and stars occurs as matter comes together via gravity, leading to the condensation and swirling of gas clouds.
• Galaxies attract and collide with each other, driving the ongoing formation of new stars and galaxies.
• There is a balance between gravity pulling matter together on galaxy and galaxy cluster scales and the expansion of the universe.
• In the early universe, there was not enough time for significant galaxy formation due to the limited time for gas condensation.
• If the universe expands too much, the galaxies are pulled apart, hindering structure formation.
• The "Cosmic Noon" period, around 2 to 4 billion years after the universe's beginning, represents a sweet spot where galaxy formation is optimal.
• The expansion of the universe has not yet pulled structures too far apart, allowing for substantial condensation and structure formation.

Reviewing Redshift & Introducing the Hubble Tension (6m5s)

• Astronomers can observe the past universe by studying the light from distant galaxies.
• Redshift measures the stretching of light from distant objects due to the universe's expansion.
• The scale factor (a) indicates the universe's size at a given time, with a = 1 today.
• Higher redshifts correspond to smaller universe sizes and earlier time periods.
• Redshift values relate to specific times, such as redshift 1 representing half the current universe size and an age of about 6 billion years.
• Determining the physical distance of objects in an expanding universe is complex due to the lack of an objective distance measure.
• Time is also not objective and varies based on factors like motion and location.
• The lookback time of distant objects is estimated using models of the universe's evolution.
• The universe's age is derived from cosmological parameters, including the current expansion rate, which is debated as the Hubble tension.
• Redshift helps measure the distance to faraway objects and provides information about the universe's expansion.
• Calculating the universe's age is challenging due to the Hubble tension, arising from different methods of measuring the expansion rate.

Cosmic Noon & Galactic Archaeology (19m14s)

• Cosmic noon, the peak of star formation, occurred approximately 2 to 3 billion years after the Big Bang.
• The Milky Way galaxy formed in the first billion years of the universe and has since consumed numerous smaller galaxies, as evidenced by stellar streams in its halo.
• Galactic archaeology studies the history of the Milky Way using stellar streams and chemical abundances of stars.
• The Gaia satellite's 3D map of the Milky Way has revealed stellar streams and provided insights into past star formation events and the galaxy's shape, including a possible warp caused by a galaxy collision.
• A recent study suggests that a galaxy collision might have triggered the formation of our solar system.
• Philosophers of physics discuss the difference between observations impossible in principle and those impossible based on current technology, using dark matter and stars as examples.
• Spectroscopy revolutionized our understanding of distant stars' chemical composition by analyzing their light.
• Dark Matter may have unknown particle interactions or require more advanced technology for direct observation.
• The arrow of time points towards increasing entropy, disorder, and ultimately, death.
• Cosmologists can directly observe different times in the universe's history through the way light travels, allowing them to witness the whole rather than just imagining it from a small amount of remains.

The Universe's Rate of Expansion (32m30s)

• The expansion of the universe creates more space between gravitationally bound objects rather than pulling them apart.
• Astronomers in the late 1990s conducted measurements to determine the ultimate fate of the universe: whether it would eventually stop expanding and collapse or continue expanding indefinitely.
• Cosmologists measured the expansion history of the universe by observing distant supernovae and calculating their physical distances and redshifts.
• The measurements revealed that the expansion of the universe is accelerating rather than slowing down, as previously thought.
• This acceleration is attributed to a mysterious force called dark energy, which acts like a "secret cosmic wind" that pushes galaxies apart at an increasing rate.
• The existence of dark energy challenges our current understanding of the universe and requires further research and exploration.

Dark Energy & The Cosmological Constant (40m29s)

• The expansion of the universe is accelerating due to an unknown cause called dark energy.
• The cosmological constant (Lambda) was initially introduced by Einstein to prevent the universe from collapsing but was later removed when it was discovered that the universe is expanding.
• The cosmological constant is now thought to be a form of vacuum energy or dark energy that is inherent to space itself.
• The smallness of the cosmological constant has led to the concept of fine-tuning, suggesting that the universe may have been precisely calibrated to allow for the existence of complex structures like galaxies and life.
• Anthropic arguments suggest that the universe's observed properties may be biased by our existence as observers, but it is unclear whether they can be applied to explain the existence of dark energy or the cosmological constant.

Poetry in the Astrophysics Podcast (55m58s)

• E.E. Cummings' poem "I carry your heart with me" contains the line "this is the Wonder that's keeping the Stars apart".
• The speaker wonders if this could be a reference to Dark Energy, which was not known about in 1952 when the poem was written.
• Einstein had proposed the cosmological constant, which is similar to Dark Energy, but later rejected it.
• The speaker suggests that the cosmological constant may have entered the public consciousness in some way, inspiring Cummings' line.

Outro (58m7s)

• The episode ends with an appreciation for Galactic archaeology and a sense of wonder towards Dark Energy as a possible explanation for the acceleration of the universe's expansion.
• The Hubble tension, which refers to the discrepancy between different measurements of the universe's expansion rate, is mentioned.
• The episode highlights how big questions about the universe can lead to different conclusions, with some turning to theological explanations and others finding scientific explanations.
• The speaker expresses a personal connection to the poetic aspects of scientific concepts.
• The next episode will discuss the astrophysics of life and how life on Earth is possible.