Elena Fernández-García
@elenafernandez.bsky.social
🌍From Spain🇪🇸
🎓PhD candidate at the Instituto de Astrofísica de Andalucía (IAA-CSIC)
🔭DESI member
🎓PhD candidate at the Instituto de Astrofísica de Andalucía (IAA-CSIC)
🔭DESI member
In the end, we’re not just watching the universe.
We’re rebuilding it—
particle by particle, law by law.
And if a universe like ours keeps emerging…
that’s one of the strongest hints we’re getting reality right. 🌌
We’re rebuilding it—
particle by particle, law by law.
And if a universe like ours keeps emerging…
that’s one of the strongest hints we’re getting reality right. 🌌
January 27, 2026 at 8:51 AM
In the end, we’re not just watching the universe.
We’re rebuilding it—
particle by particle, law by law.
And if a universe like ours keeps emerging…
that’s one of the strongest hints we’re getting reality right. 🌌
We’re rebuilding it—
particle by particle, law by law.
And if a universe like ours keeps emerging…
that’s one of the strongest hints we’re getting reality right. 🌌
Why this matters:
• Test dark matter theories
• Probe dark energy
• Predict what future telescopes should see
• Explore “what-if” universes we can’t observe
Simulations are cosmic laboratories.
• Test dark matter theories
• Probe dark energy
• Predict what future telescopes should see
• Explore “what-if” universes we can’t observe
Simulations are cosmic laboratories.
January 27, 2026 at 8:51 AM
Why this matters:
• Test dark matter theories
• Probe dark energy
• Predict what future telescopes should see
• Explore “what-if” universes we can’t observe
Simulations are cosmic laboratories.
• Test dark matter theories
• Probe dark energy
• Predict what future telescopes should see
• Explore “what-if” universes we can’t observe
Simulations are cosmic laboratories.
How do we know they work?
We compare them to reality.
• Galaxy shapes
• Large-scale clustering
• Chemical abundances
• Black hole growth
If simulations match observations → confidence grows.
If not → back to the equations.
We compare them to reality.
• Galaxy shapes
• Large-scale clustering
• Chemical abundances
• Black hole growth
If simulations match observations → confidence grows.
If not → back to the equations.
January 27, 2026 at 8:51 AM
How do we know they work?
We compare them to reality.
• Galaxy shapes
• Large-scale clustering
• Chemical abundances
• Black hole growth
If simulations match observations → confidence grows.
If not → back to the equations.
We compare them to reality.
• Galaxy shapes
• Large-scale clustering
• Chemical abundances
• Black hole growth
If simulations match observations → confidence grows.
If not → back to the equations.
Famous simulations you might’ve heard of:
• Millennium
• Illustris / IllustrisTNG
• EAGLE
• Bolshoi
• Uchuu
Each runs on millions of CPU hours—
entire universes born in silicon.
• Millennium
• Illustris / IllustrisTNG
• EAGLE
• Bolshoi
• Uchuu
Each runs on millions of CPU hours—
entire universes born in silicon.
January 27, 2026 at 8:51 AM
Famous simulations you might’ve heard of:
• Millennium
• Illustris / IllustrisTNG
• EAGLE
• Bolshoi
• Uchuu
Each runs on millions of CPU hours—
entire universes born in silicon.
• Millennium
• Illustris / IllustrisTNG
• EAGLE
• Bolshoi
• Uchuu
Each runs on millions of CPU hours—
entire universes born in silicon.
No simulation can model everything perfectly.
So physicists use subgrid models:
educated approximations for processes too small to resolve.
This is where art meets science—
and where debates get heated.
So physicists use subgrid models:
educated approximations for processes too small to resolve.
This is where art meets science—
and where debates get heated.
January 27, 2026 at 8:51 AM
No simulation can model everything perfectly.
So physicists use subgrid models:
educated approximations for processes too small to resolve.
This is where art meets science—
and where debates get heated.
So physicists use subgrid models:
educated approximations for processes too small to resolve.
This is where art meets science—
and where debates get heated.
Then comes the hard part: baryonic physics.
Gas cooling
Star formation
Supernova explosions
Black hole feedback
These processes happen on tiny scales—
yet they shape galaxies on cosmic scales.
That’s computational pain.
Gas cooling
Star formation
Supernova explosions
Black hole feedback
These processes happen on tiny scales—
yet they shape galaxies on cosmic scales.
That’s computational pain.
January 27, 2026 at 8:51 AM
Then comes the hard part: baryonic physics.
Gas cooling
Star formation
Supernova explosions
Black hole feedback
These processes happen on tiny scales—
yet they shape galaxies on cosmic scales.
That’s computational pain.
Gas cooling
Star formation
Supernova explosions
Black hole feedback
These processes happen on tiny scales—
yet they shape galaxies on cosmic scales.
That’s computational pain.
The backbone of these simulations is dark matter.
Why?
Because it dominates gravity.
Invisible clumps form first, creating a cosmic scaffold—
the cosmic web of filaments and voids.
Galaxies later light up where dark matter piles up.
Why?
Because it dominates gravity.
Invisible clumps form first, creating a cosmic scaffold—
the cosmic web of filaments and voids.
Galaxies later light up where dark matter piles up.
January 27, 2026 at 8:51 AM
The backbone of these simulations is dark matter.
Why?
Because it dominates gravity.
Invisible clumps form first, creating a cosmic scaffold—
the cosmic web of filaments and voids.
Galaxies later light up where dark matter piles up.
Why?
Because it dominates gravity.
Invisible clumps form first, creating a cosmic scaffold—
the cosmic web of filaments and voids.
Galaxies later light up where dark matter piles up.
Simulations begin just after the Big Bang.
We feed in:
• Tiny density fluctuations (from the CMB)
• Dark matter and normal matter
• Expansion of space
• Gravity
From almost nothing… structure grows.
We feed in:
• Tiny density fluctuations (from the CMB)
• Dark matter and normal matter
• Expansion of space
• Gravity
From almost nothing… structure grows.
January 27, 2026 at 8:51 AM
Simulations begin just after the Big Bang.
We feed in:
• Tiny density fluctuations (from the CMB)
• Dark matter and normal matter
• Expansion of space
• Gravity
From almost nothing… structure grows.
We feed in:
• Tiny density fluctuations (from the CMB)
• Dark matter and normal matter
• Expansion of space
• Gravity
From almost nothing… structure grows.
The goal is simple (and insane):
Start with the early universe…
apply the laws of physics…
and see if a universe like ours emerges.
If it doesn’t?
Our theories are wrong.
Start with the early universe…
apply the laws of physics…
and see if a universe like ours emerges.
If it doesn’t?
Our theories are wrong.
January 27, 2026 at 8:51 AM
The goal is simple (and insane):
Start with the early universe…
apply the laws of physics…
and see if a universe like ours emerges.
If it doesn’t?
Our theories are wrong.
Start with the early universe…
apply the laws of physics…
and see if a universe like ours emerges.
If it doesn’t?
Our theories are wrong.
So does the universe have a center?
Yes—
✨ from your perspective.
And from every other galaxy’s perspective too.
That’s not a flaw in our understanding.
It’s one of the deepest truths about reality itself. 🌌
Yes—
✨ from your perspective.
And from every other galaxy’s perspective too.
That’s not a flaw in our understanding.
It’s one of the deepest truths about reality itself. 🌌
January 26, 2026 at 6:33 PM
So does the universe have a center?
Yes—
✨ from your perspective.
And from every other galaxy’s perspective too.
That’s not a flaw in our understanding.
It’s one of the deepest truths about reality itself. 🌌
Yes—
✨ from your perspective.
And from every other galaxy’s perspective too.
That’s not a flaw in our understanding.
It’s one of the deepest truths about reality itself. 🌌
Bottom line:
• The universe has no edge we can point to
• No central location to stand on
• No privileged observer
Every observer gets the same cosmic view.
• The universe has no edge we can point to
• No central location to stand on
• No privileged observer
Every observer gets the same cosmic view.
January 26, 2026 at 6:33 PM
Bottom line:
• The universe has no edge we can point to
• No central location to stand on
• No privileged observer
Every observer gets the same cosmic view.
• The universe has no edge we can point to
• No central location to stand on
• No privileged observer
Every observer gets the same cosmic view.
Could there still be a center outside the universe?
Maybe—but that’s philosophy, not physics.
Our theories only describe the universe from within.
Asking what’s “outside” may be like asking what’s north of the North Pole.
Maybe—but that’s philosophy, not physics.
Our theories only describe the universe from within.
Asking what’s “outside” may be like asking what’s north of the North Pole.
January 26, 2026 at 6:33 PM
Could there still be a center outside the universe?
Maybe—but that’s philosophy, not physics.
Our theories only describe the universe from within.
Asking what’s “outside” may be like asking what’s north of the North Pole.
Maybe—but that’s philosophy, not physics.
Our theories only describe the universe from within.
Asking what’s “outside” may be like asking what’s north of the North Pole.
So why does it feel like we’re central?
Because the laws of physics look nearly the same in every direction.
This is called the Cosmological Principle:
On large scales, the universe is homogeneous and isotropic.
No preferred directions.
No preferred locations.
Because the laws of physics look nearly the same in every direction.
This is called the Cosmological Principle:
On large scales, the universe is homogeneous and isotropic.
No preferred directions.
No preferred locations.
January 26, 2026 at 6:33 PM
So why does it feel like we’re central?
Because the laws of physics look nearly the same in every direction.
This is called the Cosmological Principle:
On large scales, the universe is homogeneous and isotropic.
No preferred directions.
No preferred locations.
Because the laws of physics look nearly the same in every direction.
This is called the Cosmological Principle:
On large scales, the universe is homogeneous and isotropic.
No preferred directions.
No preferred locations.
That means there is no point you can travel to and say:
“This is where it all began.”
Every region of space traces back to the same early, hot, dense state.
The beginning is in the past—not in a place.
“This is where it all began.”
Every region of space traces back to the same early, hot, dense state.
The beginning is in the past—not in a place.
January 26, 2026 at 6:33 PM
That means there is no point you can travel to and say:
“This is where it all began.”
Every region of space traces back to the same early, hot, dense state.
The beginning is in the past—not in a place.
“This is where it all began.”
Every region of space traces back to the same early, hot, dense state.
The beginning is in the past—not in a place.
What about the Big Bang?
Surely that had a center, right?
Surprisingly… no.
The Big Bang wasn’t an explosion in space.
It was an expansion of space.
It happened everywhere at once.
Surely that had a center, right?
Surprisingly… no.
The Big Bang wasn’t an explosion in space.
It was an expansion of space.
It happened everywhere at once.
January 26, 2026 at 6:33 PM
What about the Big Bang?
Surely that had a center, right?
Surprisingly… no.
The Big Bang wasn’t an explosion in space.
It was an expansion of space.
It happened everywhere at once.
Surely that had a center, right?
Surprisingly… no.
The Big Bang wasn’t an explosion in space.
It was an expansion of space.
It happened everywhere at once.
Our universe works the same way—just in 3D.
Every galaxy sees other galaxies receding.
If you were in a galaxy billions of light-years away,
you’d also conclude:
👉 “Everything is moving away from me.”
No special location required.
Every galaxy sees other galaxies receding.
If you were in a galaxy billions of light-years away,
you’d also conclude:
👉 “Everything is moving away from me.”
No special location required.
January 26, 2026 at 6:33 PM
Our universe works the same way—just in 3D.
Every galaxy sees other galaxies receding.
If you were in a galaxy billions of light-years away,
you’d also conclude:
👉 “Everything is moving away from me.”
No special location required.
Every galaxy sees other galaxies receding.
If you were in a galaxy billions of light-years away,
you’d also conclude:
👉 “Everything is moving away from me.”
No special location required.
Think of the universe like the surface of a balloon. 🎈
As the balloon inflates, every point moves away from every other point.
From any dot on the surface, it looks like you’re at the center.
Yet… the surface has no center on it.
As the balloon inflates, every point moves away from every other point.
From any dot on the surface, it looks like you’re at the center.
Yet… the surface has no center on it.
January 26, 2026 at 6:33 PM
Think of the universe like the surface of a balloon. 🎈
As the balloon inflates, every point moves away from every other point.
From any dot on the surface, it looks like you’re at the center.
Yet… the surface has no center on it.
As the balloon inflates, every point moves away from every other point.
From any dot on the surface, it looks like you’re at the center.
Yet… the surface has no center on it.
Let’s start with what we know.
The universe is expanding.
But here’s the key idea most people miss:
It’s not expanding into space.
Space itself is expanding.
That changes everything.
The universe is expanding.
But here’s the key idea most people miss:
It’s not expanding into space.
Space itself is expanding.
That changes everything.
January 26, 2026 at 6:33 PM
Let’s start with what we know.
The universe is expanding.
But here’s the key idea most people miss:
It’s not expanding into space.
Space itself is expanding.
That changes everything.
The universe is expanding.
But here’s the key idea most people miss:
It’s not expanding into space.
Space itself is expanding.
That changes everything.
So is dark energy a fatal error?
Or the doorway to deeper laws of nature?
We don’t know yet.
But one thing is clear:
The universe is telling us something important.
And we’re only just learning how to listen. 🌌
Or the doorway to deeper laws of nature?
We don’t know yet.
But one thing is clear:
The universe is telling us something important.
And we’re only just learning how to listen. 🌌
January 24, 2026 at 9:47 PM
So is dark energy a fatal error?
Or the doorway to deeper laws of nature?
We don’t know yet.
But one thing is clear:
The universe is telling us something important.
And we’re only just learning how to listen. 🌌
Or the doorway to deeper laws of nature?
We don’t know yet.
But one thing is clear:
The universe is telling us something important.
And we’re only just learning how to listen. 🌌
If dark energy is wrong:
• Our understanding of spacetime changes
• The fate of the universe changes
• The foundations of modern cosmology change
If dark energy is real:
• We’ve discovered a new fundamental property of reality
Either way, we win… intellectually.
• Our understanding of spacetime changes
• The fate of the universe changes
• The foundations of modern cosmology change
If dark energy is real:
• We’ve discovered a new fundamental property of reality
Either way, we win… intellectually.
January 24, 2026 at 9:47 PM
If dark energy is wrong:
• Our understanding of spacetime changes
• The fate of the universe changes
• The foundations of modern cosmology change
If dark energy is real:
• We’ve discovered a new fundamental property of reality
Either way, we win… intellectually.
• Our understanding of spacetime changes
• The fate of the universe changes
• The foundations of modern cosmology change
If dark energy is real:
• We’ve discovered a new fundamental property of reality
Either way, we win… intellectually.
Recent observations are adding fuel to the fire.
The “Hubble tension” shows different methods give conflicting expansion rates.
Not a small discrepancy—one that keeps getting stronger.
Could this be the first crack in the standard cosmological model?
The “Hubble tension” shows different methods give conflicting expansion rates.
Not a small discrepancy—one that keeps getting stronger.
Could this be the first crack in the standard cosmological model?
January 24, 2026 at 9:47 PM
Recent observations are adding fuel to the fire.
The “Hubble tension” shows different methods give conflicting expansion rates.
Not a small discrepancy—one that keeps getting stronger.
Could this be the first crack in the standard cosmological model?
The “Hubble tension” shows different methods give conflicting expansion rates.
Not a small discrepancy—one that keeps getting stronger.
Could this be the first crack in the standard cosmological model?
Enter new physics.
Ideas on the table:
• Modified gravity (Einstein, but with a twist)
• Dynamic fields that evolve over time
• Extra dimensions
• Breakdowns of general relativity at large scales
Each option rewrites textbooks.
Ideas on the table:
• Modified gravity (Einstein, but with a twist)
• Dynamic fields that evolve over time
• Extra dimensions
• Breakdowns of general relativity at large scales
Each option rewrites textbooks.
January 24, 2026 at 9:47 PM
Enter new physics.
Ideas on the table:
• Modified gravity (Einstein, but with a twist)
• Dynamic fields that evolve over time
• Extra dimensions
• Breakdowns of general relativity at large scales
Each option rewrites textbooks.
Ideas on the table:
• Modified gravity (Einstein, but with a twist)
• Dynamic fields that evolve over time
• Extra dimensions
• Breakdowns of general relativity at large scales
Each option rewrites textbooks.
This has led some scientists to ask a dangerous question:
👉 What if dark energy doesn’t exist?
What if:
• Our distance measurements are biased
• Supernova physics is misunderstood
• Or gravity itself changes over cosmic scales
👉 What if dark energy doesn’t exist?
What if:
• Our distance measurements are biased
• Supernova physics is misunderstood
• Or gravity itself changes over cosmic scales
January 24, 2026 at 9:47 PM
This has led some scientists to ask a dangerous question:
👉 What if dark energy doesn’t exist?
What if:
• Our distance measurements are biased
• Supernova physics is misunderstood
• Or gravity itself changes over cosmic scales
👉 What if dark energy doesn’t exist?
What if:
• Our distance measurements are biased
• Supernova physics is misunderstood
• Or gravity itself changes over cosmic scales