How the universe began—and how it is likely to end | Prof Carlos Frenk | TEDxDurham

The Expanding Universe, Dark Energy, and the Future of Cosmology

Key Concepts:

• Hubble’s Law: The observation that galaxies are receding from us at a speed proportional to their distance.
• Big Bang: The prevailing cosmological model for the universe, originating from an extremely hot, dense state approximately 13.7 billion years ago.
• Cosmic Microwave Background (CMB): The residual radiation from the early universe, observed as microwaves.
• Dark Energy: A hypothetical form of energy that permeates all of space and accelerates the expansion of the universe.
• Dark Matter: A non-luminous form of matter that interacts gravitationally but does not emit or absorb light, comprising approximately 25% of the universe.
• Lambda-CDM Model: The standard model of cosmology, incorporating a cosmological constant (Lambda) representing dark energy and Cold Dark Matter (CDM).
• Cosmic Web: The large-scale structure of the universe, characterized by filaments and voids of galaxies.
• Big Rip, Heat Death, Big Crunch: Potential scenarios for the ultimate fate of the universe, dependent on the behavior of dark energy.

The Discovery of an Expanding Universe

The presentation begins with the foundational observation that the universe is expanding. This was first noted by Georges Lemaître, a Belgian priest who published his findings in French approximately 100 years ago, initially receiving little attention. Edwin Hubble later independently rediscovered this phenomenon, leading to what is now known as Hubble’s Law – though the International Astronomical Union has recently renamed it simply “Hubble law.” This law states that the velocity of a galaxy’s recession is directly proportional to its distance from us. Importantly, this expansion is not centered on Earth; any observer in any galaxy would observe the same effect. The logical consequence of an expanding universe is that it was smaller in the past, ultimately originating from an incredibly dense and hot state known as the Big Bang.

Evidence for the Big Bang: The Cosmic Microwave Background

The Big Bang theory, a concept rooted in physics and therefore requiring testable evidence, is supported by the Cosmic Microwave Background (CMB). As the universe expanded, it cooled. Approximately 350,000 years after the Big Bang (analogous to a one-day-old baby in human terms), the universe became transparent, releasing the first light. This light, having traveled for 13.4 billion years, was serendipitously discovered by Arno Penzias and Bob Wilson while attempting to study the sun using a radio telescope. They initially interpreted the pervasive “hiss” as noise, but it was, in fact, the CMB – a remnant of the Big Bang, now appearing as microwaves due to the universe’s expansion and cooling.

The Accelerating Expansion and Dark Energy

At the turn of the last century, a more perplexing discovery emerged: the expansion of the universe is accelerating. This means that the rate at which galaxies are receding from each other is increasing over time. To account for this acceleration, physicists have posited the existence of “dark energy,” a mysterious force opposing gravity. As is common in physics, when something is not understood, it is given a name – in this case, “dark energy” – to facilitate research and funding.

The Composition of the Universe: A Bizarre Reality

Current understanding of the universe’s composition is striking. Only 5% of the universe consists of ordinary matter – the atoms that make up stars, planets, and ourselves. A significant 25% is attributed to dark matter, a non-luminous substance detectable only through its gravitational effects. However, the dominant component, approximately 68%, is dark energy. Dark matter is thought to be composed of particles created shortly after the Big Bang, referred to as “cold dark matter.”

The Lambda-CDM Model and its Testing

The prevailing cosmological model, Lambda-CDM, incorporates both dark energy (represented by the cosmological constant, Lambda) and cold dark matter (CDM). This model has been extensively tested using computer simulations that model the evolution of the universe based on the laws of physics. These simulations demonstrate how dark matter collapses into “halos” within which galaxies form, creating the large-scale structure known as the cosmic web. The simulations can now accurately reproduce observed galactic structures, to the point of being indistinguishable from real astronomical images.

Challenging the Standard Model: New Data from DESI

For 25 years, the Lambda-CDM model has been remarkably consistent with observational data. However, recent data from the Dark Energy Spectroscopic Instrument (DESI) – a telescope capable of measuring distances to over 40 million galaxies with 1% precision – is challenging this model. The data suggests that the expansion history of the universe may not align with the predictions of Lambda-CDM, indicating a potential flaw in our understanding of dark energy. This finding was highlighted in publications like the New York Times and the BBC.

The Future of the Universe: Potential Scenarios

The ultimate fate of the universe depends on the behavior of dark energy. Three potential scenarios are outlined:

• The Big Rip: If dark energy increases over time, the universe will be torn apart.
• The Heat Death: If dark energy remains constant (as predicted by the cosmological constant), the universe will continue expanding indefinitely, eventually becoming a cold, desolate expanse.
• The Big Crunch: If dark energy decreases over time, the expansion will slow, stop, and eventually reverse, leading to a collapse of the universe.

Preliminary data suggests that dark energy may be declining, making the Big Crunch a more plausible, and philosophically appealing, outcome. However, this is still speculative and will require further investigation. The presenter notes that this potential shift in understanding is akin to discovering a flaw in a grand cathedral – a sign that fundamental revisions to our understanding of the universe may be necessary.

This discovery, while tentative, signals a need for a new paradigm in cosmology and opens exciting avenues for future research. The presenter concludes by emphasizing that while the answers remain elusive, the pursuit of understanding the universe is a continuous and evolving process.