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What is the UniverseMade of?

•MaterialsMagnetic Car and track set up with two weightsand several thin boards

•IntroductionProtons, Neutrons and Electrons: The Stuffof Life

You, this computer, the air we breathe, and thedistant stars are all made up of protons, neu-trons and electrons. Protons and neutrons arebound together into nuclei and atoms are nucleisurrounded by a full complement of electrons.Hydrogen is composed of one proton and oneelectron. Helium is composed of two protons,two neutrons and two electrons. Carbon is com-posed of six protons, six neutrons and six elec-trons. Heavier elements, such as iron, lead anduranium, contain even larger numbers of pro-tons, neutrons and electrons. Astronomers liketo call all material made up of protons, neutronsand electrons “baryonic matter”.

Until about twenty years ago, astronomersthought that the universe was composed almostentirely of this “baryonic matter”, ordinary stuff.However, in the past decade, there has beenever more evidence accumulating that suggeststhere is something in the universe that we cannot see, perhaps some new form of matter.

The Dark Matter Mystery

By measuring the motions of stars and gas,astronomers can “weigh” galaxies. In our ownsolar system, we can use the velocity of theEarth around the Sun to measure the Sun’s mass.The Earth moves around the Sun at 30 kilome-ters per second (roughly sixty thousand milesper hour). If the Sun were four times moremassive, then the Earth would need to movearound the Sun at 60 kilometers per second inorder for it to stay on its orbit. The Sun movesaround the Milky Way at 225 kilometers persecond. We can use this velocity (and the veloc-ity of other stars) to measure the mass of ourGalaxy. Similarly, radio and optical observationsof gas and stars in distant galaxies enable as-tronomers to determine the distribution of mass

in these systems.

The mass that astronomers infer for galaxiesincluding our own is roughly ten times largerthan the mass that can be associated with stars,gas and dust in a Galaxy. This mass discrepancyhas been confirmed by observations of gravita-tional lensing, the bending of light predicted byEinstein’s theory of general relativity.

HST Image of a gravitational lens

By measuring how the background galaxies aredistorted by the foreground cluster, astronomerscan measure the mass in the cluster. The massin the cluster is more than five times larger thanthe inferred mass in visible stars, gas and dust.

Candidates for the Dark Matter

What is the nature of the “dark matter”, thismysterious material that exerts a gravitationalpull, but does not emit nor absorb light? As-tronomers do not know.

There are a number of plausible speculations onthe nature of the dark matter:• Brown Dwarfs: if a star’s mass is less thanone twentieth of our Sun, its core is not hotenough to burn either hydrogen or deuterium,so it shines only by virtue of its gravitationalcontraction. These dim objects, intermediatebetween stars and planets, are not luminousenough to be directly detectable by our tele-scopes. Brown Dwarfs and similar objects havebeen nicknamed MACHOs (MAssive CompactHalo Objects) by astronomers. These MACHOsare potentially detectable by gravitational lens-ing experiments. If the dark matter is mademostly of MACHOs, then it is likely that bary-onic matter does make up most of the mass ofthe universe.• Supermassive Black Holes: these are thoughtto power distant quasars. Some astronomersspeculate that there may be copious numbersof black holes comprising the dark matter. These

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black holes are also potentially detectablethrough their lensing effects.• New forms of matter: particle physicists, sci-entists who work to understand the fundamen-tal forces of nature and the composition of mat-ter, have speculated that there are new forcesand new types of particles. One of the primarymotivations for building “supercolliders” is totry to produce this matter in the laboratory. Sincethe universe was very dense and hot in the earlymoments following the Big Bang, the universeitself was a wonderful particle accelerator. Cos-mologists speculate that the dark matter maybe made of particles produced shortly after theBig Bang. These particles would be very differ-ent from ordinary ?baryonic matter?. Cosmolo-gists call these hypothetical particles WIMPs (forWeakly Interacting Massive Particles) or “non-baryonic matter”.

WMAP and Dark Matter

By making accurate measurements of the cos-mic microwave background fluctuations,WMAP is able to measure the basic parametersof the Big Bang model including the densityand composition of the universe. WMAP mea-sures the density of baryonic and non-baryonicmatter to an accuracy of better than 5%. It isalso able to determine some of the properties ofthe non-baryonic matter: the interactions of thenon-baryonic matter with itself, its mass and itsinteractions with ordinary matter all affect thedetails of the cosmic microwave backgroundfluctuation spectrum.

WMAP determined that the universe is flat,meaning it is neither open nor closed, but rightbetween. A closed, or elliptical, universe expe-riences a continuous cycle of a Big Bang, ex-panding a finite amount, and falling back onitself. On the other hand, an open, or hyper-bolic, universe will expand to infinity quickly. Ittakes a flat universe, as ours is, an infiniteamount of time to extend to infinity. A universewill be elliptical, flat, or hyperbolic based uponits energy density. Remember, matter is energy:E=mc2. Of the total energy density of the Uni-verse, we now know the breakdown to be:

* 4% Atoms, 23% Cold Dark Matter, 73% DarkEnergy. Thus 96% of the energy density in theuniverse is in a form that has never been di-rectly detected in the laboratory. The actualdensity of atoms is equivalent to roughly 1 pro-ton per 4 cubic meters.

* The data places new constraints on the DarkEnergy. It seems more like a “cosmological con-stant” than a negative-pressure energy fieldcalled “quintessence”. But quintessence is notruled out.

* Fast moving neutrinos do not play any majorrole in the evolution of structure in the universe.They would have prevented the early clumpingof gas in the universe, delaying the emergenceof the first stars, in conflict with the new WMAPdata.

For more information about dark matter and darkenergy, see the WMAP website:http://map.gsfc.nasa.gov/

•ProcedureIn this lab, you will explore a model for theUniverse where you can change the mass and“dark energy” of the system. The mass in themodel corresponds to the mass of the Universe,both baryonic and non-baryonic. The incline ofthe track corresponds to the dark energy. Darkenergy is the energy that is pushing the uni-verse outward. As stated above, dark energyacts like negative pressure, like the air in theballoon in the Expansion of the Universe lab.Due to conservation of energy, we know that allof the Universes’ energy (atoms, cold dark mat-ter, and dark energy) was present at the BigBang. We refer to the energy which is expand-ing the Universe as “dark” because we have noidea what it is!

It is useful to think about dark energy in termsof anti-gravity. If there is a great abundanceof dark energy in a universe, then the universewill be open - it will expand to infinity in lessthan an infinite amount of time. This is due tothe great amount of energy pushing the matteroutward. In a closed universe, the amount ofdark energy is small enough that gravity willeventually take over, pulling the matter backtogether and creating another Big Bang. Thesituation directly in-between, a flat universe,has just the perfect amount of dark energy thatthe universe takes an infinite amount of time toexpand to infinity. In our model, if the track thecar runs on is at such an incline that the carruns right past a marked point, then the car/track universe is open. If the track has a greaterincline and the car stops before the markedpoint, the universe is closed and will fall backon itself. If the car stops right at the mark, thenthe model universe is flat. You will also be ableto change the mass of the car to see the effect

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1) Designate someone on your team for each ofthe following jobs:

a. The car launcher. You will obtain moreconsistent results by having the same personlaunch the car each time.

b. The stopper. This person makes surethe car doesn’t run off the end of the track.

c. The observer. The observer will call outthe farthest reached position of the front of thecar using the ruler on the track.

d. The recorder. This person will recordeach of the measurements in the chart below.

2) Start with the front of the car at a good num-ber, like 200 cm. Record this value. Do this withno incline on the track and no weight in the car.Have the car launcher do a few practice launchesso the technique is consistent. Make sure thecar doesn’t crash off the end of the track!

3) Once you have an idea of how far the cargoes with no incline and no weight (this will bethe farthest the car goes), decide on a referencemark somewhere between the starting value andthe value of the farthest travel distance. Themodel car/track universe that has the exactamount of mass and incline to be “flat” will stophere. Note: it will be tough to get the experi-ment exactly flat; you’ll just get as close as youcan. This is true of the actual Universe as well.As far as we can tell (5% accuracy), the Uni-verse is flat. However, it is basically impossiblefor the Universe to be EXACTLY flat.

4) Repeat each of the cases in the chart belowfive times and record the average in the chart.You will need to decide how much incline youneed to try based upon your selected mark.

•Questions1) True or False: In this model universe, a largerincline corresponds to more dark energy. Ex-plain.

2) How much more dark energy is in our Uni-verse as compared to energy from matter?

3) What does your answer to 2) say about theaccuracy of the model?

4) What seemed to have a greater effect in themodel, incline or mass?

5) What other aspects of the model are inaccu-rate?

6) Summarize what the model helped you learnabout the composition of the Universe.

mark = ____ cm

No incline Incline 1, = ____ cm

Incline 2, = ____ cm

Incline 3, = ____ cm

Incline 4, = ____ cm

Incline 5, = ____ cm

no added mass blocks

one added mass block

two added mass blocks

For credit, each member of the team needsto turn in a copy of the filled-in chart andanswers to the questions.