What is in our future?

The big crunch, the big loneliness, or something much worse?

SCDMStandard Cold Dark Matter Model

Assumptions:

• No curvature to the universe

• No dark energy or quintessence

Prediction:

• Gravity has been retarding the expansion of the universe since the Big Bang

• Gravity is the sole force effecting the expansion rate of the universe

What is wrong with the SCDM?

These three observations:• Type 1a Supernovae (1998)

• CMB-Cosmic Microwave Background (2001)

• Chandra X-Ray Observer (2004)

Have produced evidence that:

• The expansion of the universe is accelerating

• Something is counteracting gravity

Experimental Overview

Type 1a Supernovae appeared fainter than predicted

Supernovae with the higher redshifts appeared fainter than predicted by SCDM in an empty universe.

Experimental Overview

Cosmic Microwave Background echoes of the Big Bang

Figure 2. A CMB imaging of the last scattering epoch.

CMB imaging has been used to formulate a picture of the early universe

Experimental Overview

The intense gravitic field within clusters heats the gas up to 100 million degrees Celsius, making it only detectable via X-rays.

Figure 3. A quick journey amidst the super-hot gas of a cluster.

The Chandra X-Ray Observer is used to analyzes the gas fraction within clusters at different distances.

What causes this acceleration?

Quintessence?

Dark Energy?

Warping of Space-Time?

Figure 4. An exaggerated look at the deceleration and eventual acceleration of the expansion of the universe.

The Cosmic Triangle

One model used to explain this phenomenon introduces vacuum energy and a curvature to the universe. The triangle delineates different proportions of matter, curvature, and vacuum energy, such that they sum to 1. Other dimensions can be added, but the vacuum energy can be used to represent quintessence, dark energy, etc.

Figure 5. The Cosmic Triangle.

Curvature Models

Closed Open Flat

What is the actual ratio?Scientists believe that the current makeup of the

universe is dominated by Dark Energy

Figure 6. A pie chart delineating the makeup of the universe.

Our connection between dark energy and matter

There is conversion between dark matter and dark energy

-The Universe shortly after the big band is energy dominated. Most of this energy is dark resulting in Inflation

-As universe cools, energy can convert into matter including dark. This yields a greater gravitation effect thus slowing expansion

-As Universe expand, the avg. mass density decreases reducing the effect of gravity and allowing the vacuum energy of dark energy to dominate. This cause the universe to accelerate again.

Dark EnergyHow is the ratio of dark energy changing with time?

Figure 7. Possible outcomes for our universe.

Is Dark Energy Really the Answer?

String Theory predicts multiple dimensions. The answer might lie in

other dimensions / universes and our misunderstanding of how gravity

works

Traditional views: Dark Energy

• Einstein cosmological constant: empty space has mass/ energy 10^-26 kg/m^3

• Consistent with observations but so small

• Static: implies homogeneity

• Quintessence: Energy field: like a fog through space

• Adjusts to mass density: inverse to mass

• Fits inflation• Easier to mesh with

current models but still size concerns

Virtual Particles

• Virtual particles appear spontaneously in complete vacuum, and have a tiny lifespan

• It is believed that virtual particles exert a repelling gravitic force on each other

• This has led scientists to speculate, “How much does nothing weigh?”

• Experimental measurements for the mass of virtual particles is 10^50 times too great

Gravity

Is a 1/r^(x-1) law where x= #dimensions. As x increases the force of gravity decreases

2-d world 3-d world 4-d world

Traditional String theory• Assumes 3 dimensions are

infinite in length while the rest are made of tiny circles

• Other dimensions are small approximately equal to Planck length 10^-35m

• We don’t observe extra dimensions because scale is so small

• Thought to explain why laws of physics including gravity break down on a small scale

New ideas about String theory

• All dimensions are infinitely long

• Our traditional 3-d world is a membrane in which all other dimensions are orthogonal to.

• Traditional particles (proton, electron, etc) are open ended strings with ends attached to membrane. Thus we can’t escape into other dimensions

• The Graviton is special: Closed string like rubber band which is free to leave the membrane

The Graviton is free to escape the membrane while the electron is tied down

The GravitonQ: If the Graviton can escape into other

dimensions why do we see gravity as a 1/R^2 lawA: Only certain Graviton wavelengths can escape into other dimensions. Thus, there must be some critical distance / wavelength. At such a distance gravitons explore all possible routes and thus we see much lower gravity. Looks like gravitons just disappear. At small distances we approach the order of magnitude of the strings themselves and thus gravity also acts strange. We mainly observe middle range distances and thus see “normal” gravity

Imagine a hammer hitting a metal plate. The sound wave travels along the plate but a small part is transmitted perpendicular to the plate. Over small distances, the parallel motion dominates. However, over large distances, the energy lost is significant. This is analogous to gravity

Q: How does this apply to our Universe?• A: It all depends on the membrane. It is a mysterious

entity unto itself. Even in empty space virtual virtual particles can appear on membraneparticles can appear on membrane (probably due to quantum fluctuations). All other dimension are truly empty in a vacuum.

-Think of the membrane like a dielectric and virtual particles as having +/- energy just like +/- charge- The graviton wave is a transverse wave just like an EM waves. This means it moves virtual particles orthogonal to direction of motion.-When the wave enters perpendicular to the membrane, it moves the virtual particles so that they create a gravitational field that exactly cancels out that of the graviton-The Graviton can’t enter the membrane and thus we see no gravitational effects. However, the graviton slightly warps the membrane when it reorients the virtual particles-When the wave is parallel, there is no effect. Gravity acts like normal

No Graviton: virtual particles are randomly aligned

Perpendicular Graviton

V. Particles move and warp membrane

Parallel GravitonV. Particles feel no force and stay still

Why do we see “normal” gravity so often?

• Waves infringe on the membrane at all angles. But only only perpendicular components are canceled outperpendicular components are canceled out

• Graviton waves come in all sorts of wavelengths and Graviton waves come in all sorts of wavelengths and energiesenergies. These wavelengths are related to the distances between objects ie. when distances are large most waves have long wavelengths

• The membrane only responds to medium energy / membrane only responds to medium energy / wavelength waveswavelength waves. Thus small and large energy waves have no effect on virtual particles

• We neither live in a macro or microscopic world. Most of Most of the gravitons we deal with are middle range in the gravitons we deal with are middle range in energy and wavelength. They are trapped by the energy and wavelength. They are trapped by the membrane and we observe “normal” gravitymembrane and we observe “normal” gravity

Gravitons between Gravitons between the earth and sun try the earth and sun try to escape but most to escape but most are blocked by the are blocked by the membrane due to membrane due to medium range medium range energyenergy

The wavelengths The wavelengths between gravitons between gravitons emitted by distant emitted by distant galaxies are very small. galaxies are very small. Most gravitons are free Most gravitons are free to move into extra to move into extra dimensions. Therefore dimensions. Therefore we observed a much we observed a much weaker gravitational weaker gravitational attraction.attraction.

Why are only certain gravitons affected by the membrane?

Let’s return to the analogy of the dielectric

Tap water is a dielectric. EM Tap water is a dielectric. EM waves can redistribute +/- ionswaves can redistribute +/- ions

This is how a microwave works

However, the oceans don’t boil because only certain waves can move the molecules. Radio waves (long) and visible (short) go through water but Radio waves (long) and visible (short) go through water but microwave (medium) polarize.microwave (medium) polarize.-Long= little energy can’t move / polarizeLong= little energy can’t move / polarize-Short= too fast, particles can’t reactShort= too fast, particles can’t reactThis is what happens with the gravitons. The long wavelengths can’t move the virtual particles while the short ones are too fast for the particles too react.

What does this have to do with the Universe expanding?

• Graviton leakage dents the membrane • Since it is random and occurs in empty

space it imposes somewhat uniform curvature to the universe.

• When there is little mass to curve time-space, this effects comes to dominate

• Similar to a cosmological constant. This bending of time space acts like a vacuum energy that accelerates universe.

What is a vacuum energy?• Empty space is not really empty. It is populated by +/-

virtual particles that arise because of the uncertainty principle and quantum fluctuations. Since we can’t pinpoint position and momentum of mass or energy, particles can and do appear anywhere in space

• Just as air moves from high pressure to low pressure, space will do the same thing. The dents caused by the gravitons and virtual particles create high pressure spots that act to push the universe to expand. This contrasts with other models that argue that the vacuum energy is the virtual particles themselves

What are other ideas?What are other ideas?

• Gravitons have mass and decay over time. Thus the larger the scale the greater the deviation from “normal” gravity. But it doesn’t explain small scale deviations.

• Modify relativity to respond to space time curvature. Insignificant at first but as universe ages it dominates. But it still doesn’t explain why

• Modify gravity to act differently at very small and large lengths. But we still need dark energy

Will we ever know what is going on?

• Experimental evidence is the final arbiter.• Can we ever measure these effects in our

medium sized world?• Yes. Graviton leakage from the Earth and

Moon should cause the Moon to precess • How much?• One trillionth of a degree every month.

With current equipment, we might actually detect it.

What do you think it is?What do you think it is?

The key is no-one really knows The key is no-one really knows why the universe is expanding. why the universe is expanding.

It’s truly anybody’s guess.It’s truly anybody’s guess.

So…………