The future of the universe

Episode Summary

Executive Summary: Professor Risa Wexler explains how cosmology uses imaging and spectroscopy to map the universe in 2D and 3D, revealing its expansion, large-scale uniformity, and clumpiness. She describes how dark matter and dark energy shape galaxy formation and cosmic acceleration, and how surveys like DESI and Milky Way studies help constrain the universe’s composition and history.

Main Topics: Mapping the Universe with Imaging and Spectroscopy (Priority: 5/5): Wexler distinguishes between 2D images and 3D maps built from spectroscopy and redshift measurements, emphasizing that these tools let astronomers infer distance and look back in time. Observable Universe and Cosmic Scale (Priority: 5/5): The discussion explains that the universe is about 13.8 billion years old, appears uniform on large scales, has no known edge or center, and the observable universe is limited by the finite speed of light. Galaxy Formation and Evolution (Priority: 5/5): Wexler outlines the hierarchical growth of galaxies from early density fluctuations in dark matter and gas, with small clumps merging over time to form systems like the Milky Way. Dark Matter and Dark Energy (Priority: 5/5): The episode covers the unseen components that dominate the universe: dark matter as gravitationally acting matter of unknown particle nature, and dark energy as the driver of accelerated expansion. Large Surveys and DESI (Priority: 4/5): Wexler highlights the Dark Energy Spectroscopic Instrument (DESI) as a transformational survey producing tens of millions of redshifts to build massive 3D maps and constrain cosmic structure. The Milky Way as a Precision Laboratory (Priority: 4/5): She explains a companion project focused on the Milky Way and nearby satellite galaxies, using the local universe to infer dark matter behavior and to place our galaxy in context. Satellite Galaxies and Contextual Comparisons (Priority: 3/5): The conversation describes why measuring many Milky Way-like galaxies and their satellites is essential for interpreting the Milky Way’s formation history and distinguishing normal from unusual features.

Key Arguments: The universe can be studied as a 2D image plus a redshift-based third dimension, enabling a practical 3D map. Looking farther away means looking further back in time because light travels at a finite speed. On very large scales the universe is remarkably uniform, but on small scales it is clumpy due to early fluctuations that later grew by gravity. Most of the universe is not ordinary matter; dark matter and dark energy dominate the cosmic energy budget. Galaxy formation is hierarchical: early dense regions form first, then merge into larger systems over billions of years. Dark matter is inferred from gravitational effects, but its particle nature remains unknown and is a major open problem. Dark energy is not ordinary matter; it causes the universe’s expansion to accelerate and influences structure growth. The Milky Way can serve as a high-precision laboratory for dark matter and galaxy evolution, but its interpretation requires comparison with many similar galaxies. Large surveys like DESI dramatically improve cosmological measurements by providing vastly more redshifts than previous instruments.

Data Points: Age of the universe: 13.8 billion years - Wexler states the universe formed about 13.8 billion years ago. Lookback time to the farthest observable light: more than 13 billion years ago - She notes we can see light emitted more than 13 billion years ago. Sun-light travel time: about 8 minutes - Used as an example of finite light speed. DESI redshift count: 40–50 million redshifts - Wexler says DESI has taken spectra of roughly 40 or 50 million galaxies, stars, and quasars. DESI scale relative to prior instruments: more than a factor of 20 - She says DESI has taken spectra of more than 20 times as many objects as all previous instruments. Observable universe size: 13.8 billion light-years away (radius of the observable universe, as described) - Used to describe the limit of what light could have reached us since the beginning of the universe. Dark matter share of universe: more than 95% combined with dark energy; standard model less than 5% - Wexler says ordinary matter and the standard model account for less than 5% of the universe. Galaxy mass range of dark matter clumps: a few hundred million to 10^15 solar masses - She gives the range of dark matter halo masses where galaxies can form. Milky Way satellite galaxies: almost 60 known - She mentions the Milky Way has almost 60 known orbiting galaxies. Milky Way-like galaxy sample: 101 systems - Her project expanded from an intended 100 to 101 Milky Way-like galaxies. Satellite galaxies identified in the sample: almost 400 - Across those 101 systems, nearly 400 satellites were identified. Timing of galaxy formation onset: first few hundred million years - She says galaxies began forming within the first few hundred million years of the universe. Large Magellanic Cloud collision: 1–2 billion years ago - She notes the Milky Way had an important recent collision with the Large Magellanic Cloud.

Pivotal Quotes: "we get to ask and try to answer the biggest questions that we have" — Risa Wexler: Describing why she chose cosmology and astrophysics. "there is no edge, as far as we know, there is no edge, there is no center" — Risa Wexler: Explaining the large-scale structure and limits of the observable universe. "most of the universe is made of different stuff than you and me" — Risa Wexler: Introducing dark matter and dark energy as the dominant components of the cosmos.

Implications: The episode shows that modern cosmology is driven by massive surveys and precision local measurements. Understanding dark matter, dark energy, and galaxy histories may reshape both fundamental physics and our view of the Milky Way’s place in the universe.