In December last year, science teams at CERN's Large Hadron
Collider reported a hint of evidence
of a brand new particle, one that does not fit anywhere
within the standard model of particle physics.
If true, this would be the first clue of anything
beyond the standard model from the LHC.
Remember, the standard model is basically the periodic table
of fundamental particles and forces
that represents our entire current understanding
of the building blocks of all matter.
Even the incredible discovery of the Higgs bosun in 2012
was a confirmation of the final particle of the standard model.
And that was expected.
If this is real, it may break open entirely new pathways
to the research of the fundamentals of nature.
So what was seen?
Well, let's first think about how particle accelerators--
and especially the LHC-- work.
The LHC occupies a 27-kilometer circumference circle
beneath the Swiss-French border near Geneva.
A ring of gigantic magnets accelerates protons
to up to 0.99999999 of the speed of light
before colliding them from opposite directions.
The resulting collisions can produce temperatures
of several trillion Kelvin.
So a tiny spot for a tiny instant
resembles the state of the universe at a trillionth
of a second after the Big Bang.
Under these conditions, protons are obliterated.
Their energy is released and reshapes itself
into new particles.
Many, many weird particles come out of such a collision.
And some are so hopelessly unstable
that they decay into high energy light-- gamma rays--
before they ever reach a detector.
We only know they ever existed because the resulting gamma
radiation has an energy corresponding to the mass
of the decayed particle.
The Higgs boson was found because there
was a slight excess in gamma ray flashes
at 125 gigaelectron volts above the otherwise smooth spectrum
of gamma ray energies.
And now there's a new bump at 750 GEV.
This suggests a new particle with a mass much larger
than anything in the standard model.
When it was first spotted, this gamma ray excess
was just a little bump.
And it could have been the natural result
of random fluctuations.
Those happen all the time.
We wouldn't have heard anything about it
except that two completely separate experiments using
separate detectors-- Atlas and CMS-- both saw the same bump.
The significance of the result is still
low, at around a sigma of 1.6 for CMS, and lower for Atlas.
That's nowhere near the 5 sigma needed
for a confident discovery.
But particle physicists are completely losing their minds.
Over 300 papers have been submitted
to journals with a wide range of possible explanations.
The broad family of possibilities
include one-- dark matter.
There are theoretical ideas of particles
that could cause a bump at this energy,
and would also make pretty decent dark matter candidates.
Two-- a gigantic neutrino.
More accurately, a very mess of cousin to the neutrino,
predicted by supersymmetry and as yet unproven,
but a pretty popular extension to the standard model.
Three-- the big brother to the Higgs boson.
So, a higher energy vibration in the Higgs field.
Four-- the graviton.
A highly speculative particle responsible
for the transmission of the gravitational force.
But it's not yet even known whether such a particle
exists, or is needed to explain gravity.
Or five-- it's a composite of other smaller particles.
Just like the proton is a combination of three quarks,
this could be a much more massive combination
of several quarks and antiquarks.
There are links to references to all of these possibilities
in the description.
There's no point choosing between these options
until we verify the results.
The LHC is currently offline for upgrades,
and starts up again in June, following
a two-week delay due to a weasel chewing through a power cable.
I kid you not.
At that point, the signal will either solidify or vanish,
assuming no more attacks by cute animals.
OK
Time for the solution to the dark energy challenge question.
I'm just going to interrupt Matt here for a second.
Sorry, Matt.
He's about to go full nerd with the dark energy challenge
answer.
Before he sends you to sleep, I want
to respond to your comments on our Ice Age episode,
and make this important announcement.
On Monday, June 13, I'm doing a Reddit AMA.
We're going to talk about everything space time.
So space and time, really anything physics
or astrophysics.
We'll also talk about my own research
in astrophysics, which is the subject of a recent documentary
made by the American Museum of Natural History, linked below.
But this is an Ask Me Anything.
So the sky's the limit.
Wait.
It's not even the limit.
How does this work?
You head over to the R/Science subreddit that morning
and post your questions.
It will be the top three.
At 1:00 PM, I'll get to answering these.
And we'll see how much ground we can cover.
See you there.
OK.
Let's see what you guys had to say about our episode
on the ice ages and the Milankovitch cycles.
This ended up being a big discussion on climate change.
But that's cool.
Astrophysics does have a lot to say on the topic.
Bookmaker talks about the connection
between solar activity and Earth's climate.
And that's something we really didn't touch on in our episode.
So it's true that high sunspot activity
can increase solar irradiance.
And in the first half of the 1900s,
increasing activity did increase solar output by about 0.1
of a percent.
The consensus is that this is much smaller
than anthropogenic factors.
But even ignoring this consensus,
the correlation between solar activity and warming
stopped a while ago.
As Bookmaker suggests, solar activity is diminishing.
In fact, over the past 35 years, it's
decreased while temperatures continue to increase.
Xenion341 asks why the current warming trend doesn't quite
track the current CO2 trend.
to the same degree that it does in the paleoclimate record.
OK.
Well, temperature is following CO2.
But the response takes time.
Global average temperature has risen
by at least 0.6 degrees Celsius, probably closer to 1
degree since around 1900, following the CO2 increase.
However, the full effect takes longer,
because it depends on positive feedback cycles.
Reduced ice cover reduces albedo.
Warming oceans and melting permafrost
add more CO2 to the atmosphere.
In the paleoclimate record, we see
that an increase in temperature due to Earth's changing orbit
proceeds an increase in CO2.
So the feedback cycle can start at either end.
And there will always be a lag between the two.
But once initiated, it doesn't really
matter whether the CO2 increased first,
or the temperature increased first.
Each drives the other.
Sugarshakerfly gently asks whether we really
can be certain of the broad extent of an effect, given
that we can't perfectly accurately model every detail.
This is a tough one.
The fact is, we don't need to be able to predict
with absolute precision to be sure of the trend
and the severity of a phenomenon.
Weather prediction can usually tell you
that it will be hot tomorrow.
But it rarely nails the exact temperature.
Similarly, completely independent long-term climate
models differ in the details of their predictions.
But almost all agree that the current trends will continue.
By current trends, I mean the increasing temperatures,
more severe droughts, shifting climate zones, reduced ice
coverage, et cetera, whose current effects
are very well-documented.
Sugarshakerfly also asks why the Mesozoic was so bad.
Well, it wasn't.
It was great for the organisms that
were perfectly adapted to it.
Most of those same organisms went extinct due
to sudden and massive environmental changes, probably
including climate change due to meteor impact.
Sudden climate change is very, very bad
for ecosystems evolved for the current climate.
OK.
Back to [INAUDIBLE] Matt for the dark energy challenge answer.
Take it away.
For the main question, I asked you to figure out
how many times the universe doubled
in size after dark energy first started to show its influence.
And how many times it would double
in size in the future before matter no longer
has any significant influence on expansion.
More precisely, for how many past and future doublings
of the scale factor are they both at least 10%
of the energy content of the universe?
Let's think about a giant box of space
that is expanding with the rest of the universe.
We call this a co-moving volume.
To start with, let's ask how large
this volume was when dark energy only comprised
10% of its total energy.
So at that point in the past, we know
that this equation is true.
Right now, 70% of the energy in any volume in the universe
is in the form of dark energy.
So 70 parts dark energy and 30 parts matter.
Now that looks like this.
Back in the day, it was whatever the sum of dark energy
and matter was back then.
The overall amount of energy in the form of matter
doesn't change, because matter is spreading out
with the expanding volume.
Matter past equals matter now for a co-moving volume.
But the amount of energy in the form of dark energy
is proportional to the volume.
And the volume of the box is equal to the length
of its sides cubed.
We can scale past dark energy according
to the current dark energy content like this.
And we can just use the scale factor of the universe
as the side length of our box.
In fact, let's just use R for the ratio of past
to present scale factors.
In fact, R is actually what we want to find.
So our total energy becomes this.
Put all of this into our original equation
and rearrange, and we get this equation
for the ratio of scale factors.
Plug in our current 70 units of dark energy
and 30 units of matter, and we get
that the universe was 36% of its current size
when dark energy had a 10% contribution.
But I asked you how many doublings ago that was.
So you need to double that 0.36 about 1.5 times
to get to the current scale factor of 1.
You can take exactly the same approach
to ask when in the future matter will only
have a 10% contribution to the energy in that volume.
It will happen when the universe is larger by a factor of 1.57,
or about 0.7 of a single doubling.
Add these two numbers together, and you
get that matter and dark energy both produce
a significant effect for around two doublings.
That's tiny compared to the 100 past doublings that
have happened since the end of inflation,
and the infinite number of doublings the
will happen in the future.
For the extra credit question you
were asked to figure out the number of years
that this corresponds to.
Now to do that, you need to integrate the first Friedmann
equation over the scale factors that you
got for the main part of the question.
The answer is that overall, the two influences
will be within an order of magnitude
of each other for around 15 billion years.
Now that may sound like a lot.
But we find ourselves right in the middle of this very narrow
logarithmic window.
The solutions to both parts of the question
are linked in the description.
If your name appears on screen below me, you got this right,
and were randomly selected to receive a space time T-shirt.
If that's you, email us at PBSspacetime@gmail.com
with your address, US T-shirt size,
and let us know if you want a black hole orbit or an I'll
sign to anything I want T-shirt.
OK.
Blah, blah, math, blah.
What do I mean when I talk about this
being a cosmic coincidence?
Is there something about the tipping point
between the dominance of matter versus dark energy
that makes the universe more hospitable for life?
Or should we take this as evidence
that our simple idea of a constant dark energy density
is wrong?
Excellent questions that cosmologists
are thinking hard about.
We're come back to both of these ideas
in future episodes of "Space Time."
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