Conversations and Deliberations: Non-Standard Cosmological Epochs and Expansion Histories

The Big Picture

Imagine reading a biography where everything before chapter three is blank. You know roughly how the story ends, but the formative years remain a mystery. That’s the situation cosmologists face with the universe itself. We have a detailed picture of cosmic history stretching back to about one second after the Big Bang, when Big Bang Nucleosynthesis (BBN) forged the light atomic nuclei that set the ratios of hydrogen, helium, and lithium we observe today. Before that? Essentially nothing.

What filled that cosmic chapter? Was the early universe dominated by fast-moving radiation, as the simplest models assume? Or did some exotic ingredient alter the rate at which space expanded: a slowly decaying energy field, a swarm of tiny black holes, a peculiar frozen state called “stasis”?

These questions aren’t merely academic. The expansion history of the universe shapes everything from the clumping of dark matter into the structures that seeded galaxies, to the faint ripples in spacetime that next-generation detectors might soon pick up.

In September 2024, twenty physicists gathered in Pittsburgh for a uniquely structured workshop, designed not around polished talks but around open, free-flowing conversations. The result is a document capturing the current frontier of thinking on non-standard cosmological epochs: where the field stands, what remains uncertain, and where the most promising observational handles lie.

Key Insight: The universe’s expansion history before Big Bang Nucleosynthesis remains almost entirely unconstrained. Discussions at this workshop reveal both how wide the range of possibilities is and how close we may be to probing it observationally.

How It Works

The workshop was organized around nine thematic discussion blocks, each opened with a brief framing presentation before stepping back to let conversation flow. Topics ranged from concrete observational signatures to abstract formal theory.

The first major theme was the connection to observation: what telescopes and detectors might actually see if the early universe deviated from standard radiation domination. Participants discussed a suite of signals:

• Modifications to the matter power spectrum, the statistical fingerprint of how matter clusters on different scales
• Changes to the abundances of light elements produced during BBN
• Shifts in the stochastic gravitational-wave background, a faint hiss of gravitational waves from the early universe that space-based detectors like LISA may one day hear

A central topic was Early Matter-Dominated Eras (EMDEs), periods when some heavy, slowly-decaying particle or field dominated the universe’s energy budget before eventually decaying into radiation. During an EMDE, the universe expands more slowly than in standard radiation domination. This matters enormously for dark matter: particles that would have been diluted away survive, and small-scale structure forms differently. The imprints could show up in dwarf galaxies or dark matter streams detectable through gravitational lensing.

Discussions also grappled with scalar fields, fundamental quantum fields that can drive unusual expansion phases. Depending on how a scalar field rolls down its potential energy landscape, it can mimic matter, radiation, or something in between. Several questions kept coming up: How do you distinguish between different scalar field scenarios observationally? What theoretical constraints does string theory impose? Do top-down motivations from moduli fields (the parameters that set the shape of hidden extra dimensions in string theory) favor particular expansion histories?

One of the liveliest discussions centered on cosmological stasis, a recently proposed phenomenon where multiple particle species conspire to hold the universe in a fixed expansion rate for an extended period, even as individual species decay and replenish each other. The concept grew out of thinking about “tower states,” a cascade of increasingly massive particles predicted by string theory. Whether stasis is a generic outcome of certain theoretical frameworks or a fine-tuned curiosity remained an open debate.

Primordial black holes (PBHs) rounded out the discussion. Tiny black holes formed from density fluctuations in the very early universe, PBHs could have briefly dominated the universe’s energy density before evaporating via Hawking radiation, the slow, steady leak of energy that causes black holes to gradually shrink and disappear. A PBH-dominated epoch would leave its own gravitational wave signature and could affect dark matter production in ways distinct from other non-standard scenarios.

Why It Matters

The conversational format gives this document a quality that formal publications rarely have: candor. Participants were free to voice genuine uncertainty, disagreement, and speculation. The result reads less like a review article and more like a window into how the field actually thinks about these problems.

The open questions are real. How do we tell apart different non-standard scenarios that produce similar observational signals? What new computational tools do we need to simulate structure formation through exotic cosmological epochs? Can machine learning help search for subtle signatures in gravitational wave data or the cosmic microwave background?

The timing matters. The next decade will bring a flood of relevant data. Pulsar timing arrays, networks of ultra-precise cosmic clocks that detect passing gravitational waves, are already hinting at a gravitational wave background. LISA and the Einstein Telescope are on the horizon. Galaxy surveys like the Rubin Observatory’s LSST will map cosmic structure with unprecedented precision. The theoretical frameworks debated in Pittsburgh will face real tests.

Preparing for those tests, understanding which signatures are unique to which scenario, building reliable analysis pipelines, is urgent work. This workshop lays out the map.

Bottom Line: The universe’s first second remains cosmology’s greatest mystery, but a new generation of gravitational wave detectors and structure surveys is poised to crack it open. The theoretical territory mapped out at this Pittsburgh workshop defines exactly what signatures to look for.