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Dark Energy Isn't What We Thought—Here's How We Know
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The big takeaway
Two major cosmology experiments—DESI and DES—use completely different methods to probe dark energy and the universe's expansion. Neither confirms nor rules out the other alone, but together they eliminate impossible universes and narrow down which one we actually inhabit. Modern cosmology works by asking multiple independent questions of the universe until only one consistent answer remains.
The Two Experiments and Their Methods
DESI: Mapping the Universe in 3D
The Dark Energy Spectroscopic Instrument measures distances to tens of millions of galaxies to build the most detailed three-dimensional map of the universe ever made. It detects baryon acoustic oscillations—fossil sound wave patterns from the infant universe—which act as a cosmic ruler that expands with the universe, allowing precise measurement of expansion history.
DES: A Multi-Probe Laboratory
The Dark Energy Survey used a 570-megapixel camera on the Blanco telescope to scan 300 million galaxies over six years. Unlike DESI's single focused approach, DES employed five completely different cosmological probes: weak gravitational lensing, galaxy clustering, galaxy-galaxy lensing, Type Ia supernovae distance measurements, and baryon acoustic oscillations.
1
Weak gravitational lensing
Maps dark matter distribution
2
Galaxy clustering
Tracks structure growth over time
3
Galaxy-galaxy lensing
Connects visible galaxies to dark matter halos
4
Type Ia supernovae
Measures expansion history via distances
5
Baryon acoustic oscillations
Ancient pattern from early universe
Five complementary probes used by DES to interrogate the universe
How Modern Cosmology Actually Works
We Don't Measure Dark Energy Directly
No single observation directly measures dark energy. Instead, cosmologists observe galaxies, their distances, distortions in their shapes, supernovae flashes, and the cosmic microwave background. Each observation is influenced by unknown cosmological parameters—the proportions of dark energy, dark matter, regular matter, light, and neutrinos, plus how dark energy changes over time.
Cosmology as Elimination Game
Each variation in cosmological parameters represents an entire possible universe. Cosmology works by using observations to determine which hypothetical universes we are NOT in, repeatedly eliminating possibilities until only one universe remains—the one we actually inhabit. It's like playing cosmic 20 questions.
The Dark Energy Equation of State
Dark energy is described by two key parameters: omega-0 (baseline dark energy strength) and omega-a (rate of change of dark energy). Before dark energy's discovery, any combination of these parameters was possible. Each new observation eliminates regions of this parameter space, narrowing down which universe we're in.
Why Multiple Independent Questions Matter
The Two-Numbers Analogy
If someone thinks of two numbers between 1 and 10, asking 'what is their sum?' might yield five possible pairs. Asking 'what is their product?' yields different possibilities. Only by asking both questions can you pinpoint the exact numbers. Similarly, cosmological probes each ask different questions about the universe's physics, and only their combination reveals the true parameters.
1
Ask: What is the sum? Answer: 10 → Five possible pairs (1+9, 2+8, 3+7, 4+6, 5+5)
2
Ask: What is the product? Answer: 24 → Two possible pairs (4×6, 3×8)
3
Find intersection: Only 4 and 6 satisfy both conditions
4
Conclusion: Multiple independent questions eliminate degeneracies
How asking multiple questions narrows down the answer
Degeneracies and Why They Matter
Degeneracies are multiple plausible cosmological parameters that could produce the same observations—like how several number pairs sum to 10. A good cosmological probe constrains some parameters better than others. The ideal combination of probes minimizes degeneracies, leaving tight constraints on parameters of interest.
Covariance: The Hidden Bias Problem
Different cosmological probes are not independent; they may observe the same galaxies, use the same distance calibrations, or share assumptions about galaxy formation. These shared uncertainties, called covariance, mean combining datasets naively overcounts evidence. Cosmologists must account for how much information is genuinely new versus already counted.
What DES and DESI Actually Found
Both Experiments Favor Standard Cosmology—Slightly
Analyzed independently, both DES and DESI are beautifully consistent with the standard cosmological model where dark energy is a constant (cosmological constant). However, both experiments very slightly favor a universe where dark energy is actually diminishing over time, hinting at physics beyond the standard model.
Standard model prediction
Dark energy is constant
DES and DESI hint
Dark energy may be decreasing
Both experiments slightly favor changing dark energy over the standard constant model
Neither Experiment Confirms the Other—That's the Point
DES and DESI were not built to verify each other; they were built to interrogate the universe from different angles. Together, they eliminate more possible universes than either alone. When combined with cosmic microwave background and supernova data, they push slightly further toward favoring decreasing dark energy, but not enough to rule out standard physics.
The Exciting Result: DESI's Hints Survive
The most intriguing finding is that DES independently points to the same region of parameter space as DESI's hints about decreasing dark energy. This is crucial because it rules out bias in DESI's method. If DES had immediately disfavored DESI's hints, it would suggest DESI had a systematic error. Instead, the island of universes with decreasing dark energy remains viable.
The Future of Cosmology
The Next Decade of Universe-Culling
Four major observatories will continue asking new questions of the universe over the next decade: Rubin Observatory will discover billions of new galaxies and monitor the universe over time; Euclid is already mapping the sky with high spatial resolution; the Roman Space Telescope will perform exquisitely sharp imaging with better calibration; and DESI will expand the largest 3D map ever assembled.
1
Rubin Observatory
Billions of new galaxies, decade-long monitoring
2
Euclid
Vast sky patch with high spatial resolution
3
Roman Space Telescope
Sharp imaging with superior calibration
4
DESI continuation
Largest 3D universe map expansion
Four major observatories driving the next decade of cosmology
Science as Relentless Questioning
Modern cosmology exemplifies how humans learn about nature: we believe the world is self-consistent and that independent lines of evidence must agree on the same story. Even when we think we have the answer, we keep asking questions. The hardest part of frontier science isn't discovering something new—it's convincing ourselves we haven't fooled ourselves.
Worth quoting
"It's only in the combination of all of our cosmological probes that we find the answer we were looking for."
— Host, at [5:31]
"Cosmology is about figuring out which of these infinite possible universes we are in."
— Host, at [7:14]
"At the frontier of science, the hardest part isn't discovering something new, it's convincing ourselves that we haven't fooled ourselves."
— Host, at [17:34]
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Dark Energy Isn't What We Thought—Here's How We Know

Summary of the video “The Universe Has Been Expanding Since the Big Bang. Just Not How We Thought.” by PBS Space Time.

Two major cosmology experiments—DESI and DES—use completely different methods to probe dark energy and the universe's expansion. Neither confirms nor rules out the other alone, but together they eliminate impossible universes and narrow down which one we actually inhabit. Modern cosmology works by asking multiple independent questions of the universe until only one consistent answer remains.

The Two Experiments and Their Methods

DESI: Mapping the Universe in 3D

The Dark Energy Spectroscopic Instrument measures distances to tens of millions of galaxies to build the most detailed three-dimensional map of the universe ever made. It detects baryon acoustic oscillations—fossil sound wave patterns from the infant universe—which act as a cosmic ruler that expands with the universe, allowing precise measurement of expansion history.

DES: A Multi-Probe Laboratory

The Dark Energy Survey used a 570-megapixel camera on the Blanco telescope to scan 300 million galaxies over six years. Unlike DESI's single focused approach, DES employed five completely different cosmological probes: weak gravitational lensing, galaxy clustering, galaxy-galaxy lensing, Type Ia supernovae distance measurements, and baryon acoustic oscillations.

How Modern Cosmology Actually Works

We Don't Measure Dark Energy Directly

No single observation directly measures dark energy. Instead, cosmologists observe galaxies, their distances, distortions in their shapes, supernovae flashes, and the cosmic microwave background. Each observation is influenced by unknown cosmological parameters—the proportions of dark energy, dark matter, regular matter, light, and neutrinos, plus how dark energy changes over time.

Cosmology as Elimination Game

Each variation in cosmological parameters represents an entire possible universe. Cosmology works by using observations to determine which hypothetical universes we are NOT in, repeatedly eliminating possibilities until only one universe remains—the one we actually inhabit. It's like playing cosmic 20 questions.

The Dark Energy Equation of State

Dark energy is described by two key parameters: omega-0 (baseline dark energy strength) and omega-a (rate of change of dark energy). Before dark energy's discovery, any combination of these parameters was possible. Each new observation eliminates regions of this parameter space, narrowing down which universe we're in.

Why Multiple Independent Questions Matter

The Two-Numbers Analogy

If someone thinks of two numbers between 1 and 10, asking 'what is their sum?' might yield five possible pairs. Asking 'what is their product?' yields different possibilities. Only by asking both questions can you pinpoint the exact numbers. Similarly, cosmological probes each ask different questions about the universe's physics, and only their combination reveals the true parameters.

Degeneracies and Why They Matter

Degeneracies are multiple plausible cosmological parameters that could produce the same observations—like how several number pairs sum to 10. A good cosmological probe constrains some parameters better than others. The ideal combination of probes minimizes degeneracies, leaving tight constraints on parameters of interest.

Covariance: The Hidden Bias Problem

Different cosmological probes are not independent; they may observe the same galaxies, use the same distance calibrations, or share assumptions about galaxy formation. These shared uncertainties, called covariance, mean combining datasets naively overcounts evidence. Cosmologists must account for how much information is genuinely new versus already counted.

What DES and DESI Actually Found

Both Experiments Favor Standard Cosmology—Slightly

Analyzed independently, both DES and DESI are beautifully consistent with the standard cosmological model where dark energy is a constant (cosmological constant). However, both experiments very slightly favor a universe where dark energy is actually diminishing over time, hinting at physics beyond the standard model.

Neither Experiment Confirms the Other—That's the Point

DES and DESI were not built to verify each other; they were built to interrogate the universe from different angles. Together, they eliminate more possible universes than either alone. When combined with cosmic microwave background and supernova data, they push slightly further toward favoring decreasing dark energy, but not enough to rule out standard physics.

The Exciting Result: DESI's Hints Survive

The most intriguing finding is that DES independently points to the same region of parameter space as DESI's hints about decreasing dark energy. This is crucial because it rules out bias in DESI's method. If DES had immediately disfavored DESI's hints, it would suggest DESI had a systematic error. Instead, the island of universes with decreasing dark energy remains viable.

The Future of Cosmology

The Next Decade of Universe-Culling

Four major observatories will continue asking new questions of the universe over the next decade: Rubin Observatory will discover billions of new galaxies and monitor the universe over time; Euclid is already mapping the sky with high spatial resolution; the Roman Space Telescope will perform exquisitely sharp imaging with better calibration; and DESI will expand the largest 3D map ever assembled.

Science as Relentless Questioning

Modern cosmology exemplifies how humans learn about nature: we believe the world is self-consistent and that independent lines of evidence must agree on the same story. Even when we think we have the answer, we keep asking questions. The hardest part of frontier science isn't discovering something new—it's convincing ourselves we haven't fooled ourselves.

Notable quotes

It's only in the combination of all of our cosmological probes that we find the answer we were looking for. — Host
Cosmology is about figuring out which of these infinite possible universes we are in. — Host
At the frontier of science, the hardest part isn't discovering something new, it's convincing ourselves that we haven't fooled ourselves. — Host

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