主题：理论物理学术报告(10.20)

 

理论物理学术报告(10.20号)

 

10.20日 (星期一)

 

时间：上午9:30-10:30

地点：研究生院国际会议厅308室

报告人: R.M. Baker教授

报告题目:  <美国国家自然科学基金高频遗迹引力波探测项目介绍>

内容简介:  将介绍当前国际上高频遗迹引力波理论研究进展, 同时还将介绍美国国家自然科学基金高频遗迹引力波探测项目的进展情况。

个人简介：Baker教授为国际著名物理学家和天文学家, 现为美国奥克兰引力物理研究所资深研究员，美国高频引力波委员会主席。Baker毕业于美国加利福尼亚大学（UCLA），1959-1963年就职该校天文系，在该校获得博士学位，并被UCLA官方誉为美国航天领域一流工程博士学位。Baker曾为加州天体动力学研究中心的负责人，并担任过美国航空航天研究所天体动力学委员会主席。他曾因天体动力学方面的杰出贡献而获得了里根总统授予的Brouwer奖。Baker从上世纪60年开始致力于天体动力学和高频引力波的研究，并有多本科学专著，发表了两百多篇论文，其中引力物理论文四十余篇，并以独立作者身份在国际顶级刊物《Science》发表论文两篇。Baker教授是第一届国际高频引力波大会主席（2003年，美国，弗吉尼亚），第二届国际高频引力波会议名誉主席（2007年，美国，德克萨斯）。

欢迎对天体物理, 相对论, 引力波及引力波探测感兴趣的同学和老师积极参与! 欢迎校内外专家、学者光临!

===========================

 

时间：上午10:30-11:30

地点：研究生院国际会议厅308室

报告人: 美国费米国家加速器实验室 A.W. Beckwith博士

报告题目:  <LHC inverse problem, and how its resolution may permit us to obtain realistic Dark Matter candidates?>

内容简介:  How the LHC can enable us to get realistic dark matter (DM) candidates? We wish to state the general LHC inverse problem. Given a finite set of measurements with finite resolution, how does one map back to the underlying theory for a new physics phenomena? We will for the sake of convenience confine our remarks to DM candidates, and their linkage to parameter space issues.

个人简介：A.W. Beckwith博士现为美国费米国家加速器实验(FNAL,或称为费米国立加速器实验室)研究员,美国空间物理委员会和空间天体物理委员会研究员,美国粒子束推进研究组成员,美国高频引力波科学团队成员。其研究领域涉及高能物理,粒子物理,广义相对论,宇宙学,凝聚态物理,超弦等. 近期的工作还包括早期宇宙熵的演化效应和高频遗迹引力波效应等课题。2008年作为美国费米实验室代表参加了欧洲筹建新一代高能加速器委员会的工作.这次来访将作"欧洲大型强子对撞机LHC暗物质寻找"和"早期宇宙的熵和高频遗迹引力波"两次学术报告。 

简单提示: 理论界目前通常的观点:我们的世界95%以上由看不见的暗物质和暗能量所组成(据大爆炸理论估算,同时部分天体物理实验已表明确实存在暗物质与暗能量,其关键证据是目前观测到宇宙正在加速膨胀的事实), 因此不要被表面现象所迷惑,主宰世界的是看不见的物质.但是这些暗物质和暗能量究竟是什么仍然是一个未解之迷. Beckwith博士的内容主要结合LHC的实验条件探讨暗物质可能的候选者.

(注: 暗物质：通过研究星系转动规律证明有该类物质存在,但目前通过任何手段都无法观测到它们; 其特点是有质量并会形成巨大的团块, 暗物质团块的引力作用在使常规物质形成星系的过程中起了关键作用。暗能量：将暗物质和通常物质都考虑在内仍无法解释宇宙微波背景辐射的结果, 即宇宙在大尺度上是平坦的这一事实, 但通过引入暗能量则可以解释该观测结果; 暗能量的特点是其均匀分布在整个宇宙空间，其作用与引力相反，是一种斥力，把宇宙推散开来(从作用上来看与爱因斯坦宇宙学常数项的作用类似,因此也有人将暗能量等同于爱因斯坦宇宙学常数,但这一点还没有定论)。)

欢迎对粒子物理理论及实验感兴趣的同学参与! 欢迎校内外专家、学者光临!

 

 

                                                     2008-10-13
                                                      (理论物理研究所)

 

以下为简单的背景介绍,希望能对理解报告人的内容有所帮助.



On the Edge: Gravitational Waves

(NASA美国国家航空航天局)

 

At the NASA Goddard Space Flight Center's Laboratory for High Energy Astrophysics, a new research group is devoting their collective effort to understanding and detecting gravitational waves. Scientists are creating computer programs to model gravitational waves that will be detected by a new NASA satellite, called LISA.

 

 

General Relativity

In 1916, Albert Einstein published his famous Theory of General Relativity. His theory describes how space-time is affected by mass. We can think of space-time as a fabric that bends or curves when we place an object on it. Keep in mind that the 2-dimensional fabric analogy is just a model we use to represent what is actually 4-dimensional space-time (the normal three dimensions of space, plus a fourth dimension of time).

 

Illustration showing the effect the mass of the Sun has on space-time. 

 

Imagine pulling a sheet taut and placing a bowling ball in the center of it; you will notice that the ball produces a curve in the sheet. The curve is weak far away from the ball, and steeper near the ball. In fact, the sheet is a bit stretched in that area near the ball, as well. This situation describes the curvature of space-time, and how it is affected by mass. Near a mass, space-time curves more drastically and stretches. Near a very large mass, the 'dent' in space-time is very deep, and the stretches are near the breaking point. This means that since space-time stretches near a mass, not only is space stretched out, but so is time. What do you think would happen if you put something extremely heavy on the sheet? Obviously, you might have a hard time holding the sheet up, but imagine that you had some help from Superman. The heavy object would break through the sheet! In space, this is what we call a black hole. The mass is so large that anything that comes near it (even light) falls through the hole, and is never able to return.

 

So What is a Gravitational Wave?

 

Illustration showing two black holes orbiting each other and emitting gravitational waves. 

 

Most scientists describe gravitational waves as "ripples in space-time." Just like a boat sailing through the ocean produces waves in the water, moving masses like stars or black holes produce gravitational waves in the fabric of space-time. A more massive moving object will produce more powerful waves, and objects that move very quickly will produce more waves over a certain time period.

 

Where Do Gravitational Waves Come From?

Gravitational waves are usually produced in an interaction between two or more compact masses. Such interactions include the binary orbit of two black holes, a merge of two galaxies, or two neutron stars orbiting each other. As the black holes, stars, or galaxies orbit each other, they send out waves of "gravitational radiation" that reach the Earth, However, once the waves do get to the Earth, they are extremely weak. This is because gravitational waves, like water waves, decrease in strength as they move away from the source. Even though they are weak, the waves can travel unobstructed within the 'fabric' of space-time. This how they are able to reach the Earth and provide us with information that light cannot give.

 

 

How Can We Detect Gravitational Waves?

Artist's concept of LISA.

Since the waves are so weak when they reach us, scientists had to use their imaginations to come up with instruments sensitive enough to detect such slight variations in space-time. Interferometry is the technique astronomers use to detect small stretches in space-time. The technique requires test masses to be set at a large distance from each other. Lasers make continuous measurements of the distance between each of the test masses. The masses are free to move so that when a gravitational wave passes, the distance between the masses will fluctuate. That is, space-time will be stretched. The lasers record this variation in distance, and the scientists know that a wave has passed. The greater the distance between the masses, the more sensitive the lasers are to small fluctuations. There are currently several ground-based detectors in operation or under construction, including LIGO (USA), VIRGO (Italy/France), GEO (Germany/Great Britain), and TAMA (Japan). The space-based observatory LISA is scheduled to launch in 2011.

 

In order to detect gravitational waves, it is necessary to create a model of what the incoming waveform might look like. Since there are so many sources at a given time, scientists must create computer models of gravitational waves so they know what to look for in what seems like a huge mess of data. Dr. Joan Centrella, a theoretical astrophysicist at NASA's Goddard Space Flight Center, leads a team of scientists who create these models. Currently, the group is working on computer models of massive black hole coalescences that occur when the black holes at the centers of two colliding galaxies spiral into each other. "Once we have the models for this system, we can just substitute different masses for the black holes. That way, several models can be made from one program," says Dr. Centrella.

 

What Will We Learn From the Detectors?

Gravitational waves will help physicists and astronomers to understand some of the most fundamental laws of physics. They will also tell us about the dynamics of large-scale events in the Universe like the death of stars, and the birth of black holes. With LISA, scientists will be able to probe through space and time, to observe the Universe just a fraction of a second after the Big Bang. Using this information, we may be able to learn more about how the Universe began and evolved as well as what might be in store for the future.

 

Dark Energy

(NASA美国国家航空航天局)

 

The discovery in 1998 that the Universe is actually speeding up its expansion was a total shock to astronomers. It just seems so counter-intuitive, so against common sense. But the evidence has become convincing.

 

The evidence came from studying distant type Ia supernovae. This type of supernova results from a white dwarf star in binary system. Matter transfers from the normal star to the white dwarf until the white dwarf attains a critical mass (the Chandrasekhar limit) and undergoes a thermonuclear explosion. Because all white dwarfs achieve the same mass before exploding, they all achieve the same luminosity and can be used by astronomers as "standard candles." Thus by observing their apparent brightness, astronomers can determine their distance using the 1/r2 law.

 

By knowing the distance to the supernova, we know how long ago it occurred. In addition, the light from the supernova has been red-shifted by the expansion of the universe. By measuring this redshift from the spectrum of the supernova, astronomers can determine how much the universe has expanded since the explosion. By studying many supernovae at different distances, astronomers can piece together a history of the expansion of the universe.

 

In the 1990's two teams of astronomers, the Supernova Cosmology Project and the High-Z Supernova Search, were looking for distant type Ia supernovae in order to measure the expansion rate of the universe with time. They expected that the expansion would be slowing, which would be indicated by the supernovae being brighter than their redshifts would indicate. Instead, they found the supernovae to be fainter than expected. Hence, the expansion of the universe was accelerating!

 

In addition, measurements of the cosmic microwave background indicate that the universe has a flat geometry on large scales. Because there is not enough matter in the universe - either ordinary or dark matter - to produce this flatness, the difference must be attributed to a "dark energy". This same dark energy causes the acceleration of the expansion of the universe. In addition, the effect of dark energy seems to vary, with the expansion of the Universe slowing down and speeding up over different times.

 

Astronomers know dark matter is there by its gravitational effect on the matter that we see and there are ideas about the kinds of particles it must be made of. By contrast, dark energy remains a complete mystery. The name "dark energy" refers to the fact that some kind of "stuff" must fill the vast reaches of mostly empty space in the Universe in order to be able to make space accelerate in its expansion. In this sense, it is a "field" just like an electric field or a magnetic field, both of which are produced by electromagnetic energy. But this analogy can only be taken so far because we can readily observe electromagnetic energy via the particle that carries it, the photon.

 

Some astronomers identify dark energy with Einstein's Cosmological Constant. Einstein introduced this constant into his general relativity when he saw that his theory was predicting an expanding universe, which was contrary to the evidence for a static universe that he and other physicists had in the early 20th century. This constant balanced the expansion and made the universe static. With Edwin Hubble's discovery of the expansion of the Universe, Einstein dismissed his constant. It later became identified with what quantum theory calls the energy of the vacuum.

 

In the context of dark energy, the cosmological constant is a reservoir which stores energy. Its energy scales as the universe expands. Applied to the supernova data, it would distinguish effects due to the matter in the universe from those due to the dark energy. Unfortunately, the amount of this stored energy required is far more than observed, and would result in very rapid acceleration (so much so that the stars and galaxies would not form). Physicists have suggested a new type of matter, "quintessence," which would fill the universe like a fluid which has a negative gravitational mass. However, new constraints imposed on cosmological parameters by Hubble Space Telescope data rule out at least simple models of quintessence.

 

Other possibilities being explored are topological defects, time varying forms of dark energy, or a dark energy that does not scale uniformly with the expansion of the universe.

 

The Nature of Dark Matter

(NASA美国国家航空航天局)

 

The Coma Cluster of galaxies

In general, scientists learn about the Universe by the electromagnetic radiation (or light) that we see from it. The light we see is in the form of radio waves, infrared, optical, ultraviolet, X-ray, and gamma-ray emission. But what if there is material in the Universe that does not glow? How will we ever know it is there? How can we tell how much of it there is? How do we know what it is?

 

Such material is called "dark matter" and astronomers now believe that most of the material in the Universe is made of this stuff. It is material that does not emit sufficient light for us to directly detect it, yet there are a variety of ways that we can indirectly detect it. The most common method involves the fact that the dark matter has a gravitational pull on both the light and the sources of light that we can see. From the effects of "extra" gravity that we detect, we infer how much mass must be present.

 

The image above shows one way this is done. Pictured here are two superimposed images of the Coma Cluster of galaxies. The red areas are X-ray light seen by the Einstein satellite; the blue is visible light from a Palomar Sky Survey optical image (made with ground-based telescopes). Scientists have used these observations to determine the amount of gravity required to hold together all the mass detected in the image. Suprisingly, there is not nearly enough mass observed to explain the inferred gravity - somehow, there is undetected "missing mass." What could this "missing mass" be?

 

The kinds of materials that we experience every day are made of atoms, which are made of protons, neutrons, and electrons. We refer to this type of matter as "baryonic". Is the dark matter in the Universe made of the same stuff that we are familiar with, i.e. is it baryonic? Or is it something strange ... some kind of exotic new material, which we could call non-baryonic?

 

So far, it looks like there are both baryonic and non-baryonic types of dark matter. Some dark matter may be composed of regular matter (ie., baryonic), but simply not give off much light. Things like brown dwarf stars would be in this catagory. Other non-baryonic dark matter may be tiny, sub-atomic particles which aren't a part of "normal" matter at all. If these tiny particles have mass and are numerous, they could make up a large part of the dark matter we think exists. If true, then it's possible most of the matter in the Universe is of some mysterious form that we cannot yet even identify!