What If The Cosmological Constant Is NOT Constant?

Source: What If The Cosmological Constant Is NOT Constant?, PBS Space Time, 17:32, uploaded 2024-10-10, Watch Later position 723.

PBS Space Time revisits the assumption that dark energy has stayed constant since the beginning of the universe. The first-year results from the Dark Energy Spectroscopic Instrument (DESI) fit a changing form of dark energy better than a cosmological constant when combined with measurements from the cosmic microwave background and supernovae. The preference depends on which data sets are combined, and it remains below the threshold that would support a discovery. The episode treats the result as a reason to look again at the most basic model of cosmic expansion.

Dark energy and the vacuum problem

In 1998 astronomers found that the expansion of the universe is accelerating. General relativity describes this with a cosmological constant, usually written as Lambda. Add a constant energy density to space and the equations produce accelerating expansion. Lambda-CDM, the standard model of cosmology, takes Lambda to be constant and uses CDM for cold dark matter.

Quantum field theory supplies a candidate for this constant energy. Fluctuations in the quantum fields should contribute an energy of the vacuum, which would behave like a cosmological constant. The problem lies in the size of the contribution. The most natural calculations seem to send the vacuum energy to zero or to a value so large that the universe could not form. The small positive value we observe therefore requires extreme fine-tuning of the field parameters.

One response treats our universe as a rare habitable member of a much larger set of universes with different vacuum energies. The episode calls this an anthropic argument. Another response gives dark energy a different origin, such as an undiscovered quantum field. A field can change in strength as the universe ages, which would make the rate of acceleration change as well.

The older measurements and the Hubble tension

The first evidence for acceleration came from type Ia supernovae, exploding white dwarf stars whose distances can be inferred from their brightness. Their measurements trace the expansion across roughly the more recent half of cosmic history. That span shows that the expansion rate changes, although it leaves too little history to reveal whether the rate of change is itself changing.

The cosmic microwave background gives a second anchor at the beginning of cosmic time. Under the assumption of constant acceleration, the expansion rate inferred from the CMB comes out smaller than the rate inferred from supernovae. This disagreement is the Hubble tension. A calibration error in one of the measurements could explain it. A changing acceleration rate could also explain part of the mismatch, which gives a reason to map the expansion history with one method across as much time as possible.

Supernovae become too faint at the needed distances, whilst the CMB exists at only one extreme distance. Baryon acoustic oscillations offer a bridge between them.

The sound horizon in the galaxy distribution

In the few hundred thousand years after the Big Bang, the universe was a hot plasma without atoms or stars. Gravity pulled the plasma towards regions that would later become galaxy clusters. As the inflows met, pressure sent sound waves back out through the material and created expanding shells of higher density. Matter and radiation moved together, so the waves could travel through the early universe.

When the universe cooled enough for atoms to form, matter and light separated. The sound waves stopped moving, leaving their pattern in the distribution of matter. Galaxies later formed around the denser regions, with weaker shells surrounding them. The signal is faint. It appears as a slightly higher probability of finding two galaxies at the separation set by the old wave, rather than as visible rings around individual clusters.

That separation is the BAO sound horizon. The episode puts its present size at roughly 500 million light years after cosmic expansion has stretched it. Because the early-universe physics tells us how large the ruler should be, its apparent angular size gives a physical distance. Pair that distance with the redshift of the galaxies and the result traces the expansion history.

DESI measures the redshifts. As light crosses an expanding universe, its wavelengths stretch. DESI places up to 5,000 optical fibres on a plate, with small robots positioning the fibres at the locations of galaxies and quasars. A spectrograph separates the light into wavelengths, and the emission lines of known elements reveal the redshift. The first year of the survey measured 6 million redshifts. The full survey aims for 40 million galaxies and quasars, which would produce a three-dimensional map reaching roughly 11 billion years into the past.

What the first DESI data say

DESI’s BAO measurements alone agree closely with Lambda-CDM. In that model the dark-energy equation-of-state parameter stays at -1. The episode calls this parameter omega, whilst the DESI papers write it as $w$. The value describes the pressure associated with a given energy density.

The result changes when DESI is combined with Planck’s CMB measurements and several supernova surveys. The combined data fit a model with $w>-1$, in which dark energy becomes weaker over time, more closely than a model with constant dark energy. The size of the preference depends on the data set. DESI plus the CMB gives 2.6 sigma. Adding particular supernova samples raises the preference to between 3.5 and 3.9 sigma, with the episode describing the largest case as nearly 4 sigma.

Those figures carry two limits. A 2.6-sigma result counts as a curiosity that calls for more observations, rather than as a statistically significant detection. Even nearly 4 sigma falls short of the usual 5-sigma standard. The stronger result also depends on supernova distances, which rest on a longer chain of calibrations than the CMB measurement. Each additional step offers another place for a systematic error to enter. The episode treats the possible drift in dark energy as worth testing whilst keeping the conclusion provisional.

Thawing quintessence and the fate of the universe

The broad class of models with changing dark energy is called quintessence. In the version discussed here, thawing quintessence begins with dark energy that behaves almost like a constant and gradually becomes variable. The DESI analysis finds that this class fits the combined surveys better than constant dark energy, although the fit still describes a model preference rather than a discovery of a field.

A weakening dark energy changes the possible endings of the universe. If it later strengthens instead, it could produce a Big Rip, in which the expansion eventually tears apart bound structures and then space-time itself. The thawing picture favoured by the first DESI combination makes that reversal less likely.

The Big Crunch sits at the other end of the possibilities. Even a universe without dark energy should keep expanding under the current expansion rate, despite the gravitational pull of its galaxies. If dark energy fades to zero, expansion continues and the heat death is delayed. Recollapse requires a deeper change in the equation of state. If $w$ rises from its current value near -1 to $-1/3$ or higher, dark energy would cease to produce the negative pressure that drives acceleration and could help pull the universe back together.

A possible constraint on string theory

The episode then moves from cosmic history to the problem of testable high-energy theory. General relativity and the Standard Model have remained difficult to extend, whilst string theory allows a vast landscape of possible universes. That landscape makes it hard to identify the particular set of conditions that describes our universe.

A line of work from 2018 argued that only some of the apparently possible string-theory universes should remain stable. The episode connects that stability requirement with dark energy that decreases over time. If the DESI preference survives later observations, it could therefore look like an early observational constraint on the string landscape. This is a conditional claim about a contested theoretical programme. The measurement would still need to settle first, and the connection between a cosmological fit and a string-theory prediction would need more work than the episode can provide.

The Hubble tension remains

The changing-dark-energy explanation does not resolve the Hubble tension in the direction suggested by DESI. Reconciling the CMB and supernova expansion rates would require the acceleration to increase. DESI’s combined result points towards a decrease. Its own inferred Hubble constant also agrees more closely with the Planck CMB result than with the supernova result.

The episode therefore leaves both questions open. DESI had completed its second year of observations when the video was made, and later results were expected to arrive faster than the first-year release. The full DESI sample, the Dark Energy Survey and the Vera C. Rubin Observatory should narrow the allowed value of the equation of state. With enough independent measurements, cosmologists may learn whether Lambda is constant, whether a field drives a slow change, and which long-term futures remain physically possible.

Limits

This note follows the complete English caption track and the metadata and description supplied with the 2024 PBS Space Time episode. The description contains promotion and production credits, while it provides no direct links to the scientific papers discussed in the narration. The episode reports the DESI significance levels, the BAO scale, the survey counts and the string-theory connection without developing the underlying analyses in full. Those figures and interpretations belong to the episode’s presentation and should be read as provisional, especially because the strongest preference depends on supernova data and remains below discovery significance.

The 2018 string-theory discussion is also compressed. The related paper listed below is a likely match for the episode’s reference to string swampland work, rather than a citation supplied by PBS. The episode itself does not name the authors or title.

Further reading / references

• DESI 2024 VI: Cosmological Constraints from the Measurements of Baryon Acoustic Oscillations, the first-year DESI cosmology paper whose combinations with CMB and supernova data give the significance levels discussed in the episode.
• DESI 2024: Constraints on Physics-Focused Aspects of Dark Energy using DESI DR1 BAO Data, which studies thawing, emergent and mirage classes of evolving dark energy. The episode’s reference to thawing quintessence follows this line of analysis.
• Agrawal, Obied, Steinhardt and Vafa, “On the Cosmological Implications of the String Swampland”, a likely corresponding 2018 source for the episode’s discussion of quintessence and string-theory constraints.