Saturday, 15 December 2007

Dark Matter: Invisible, Mysterious and Perhaps Nonexistent

Dark Matter: Invisible, Mysterious and Perhaps Nonexistent
By Robert Roy Britt
Senior Science Writer
posted: 10 October 2005
04:31 pm ET

In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy.[1] Its existence is inferred from gravitational effects on visible matter and gravitational lensing of background radiation, and was originally hypothesized to account for discrepancies between calculations of the mass of galaxies, clusters of galaxies and the entire universe made through dynamical and general relativistic means, and calculations based on the mass of the visible "luminous" matter these objects contain: stars and the gas and dust of the interstellar and intergalactic medium. Many experiments to detect dark matter through non-gravitational means are underway.

Edited by:
Arip Nurahman
(Teacher and Professional Lecturer)
Guru dan Dosen Profesional

Department of Physics, Faculty of Sciences and Mathematics
Indonesia University of Education
Follower Open Course Ware at MIT-Harvard University, M.A., U.S.A.

Galaxies don't have enough regular matter to keep them from flying apart, scientists have been telling us for years. So there must be a bunch of unseen "dark matter" lurking in every galaxy.

But dark matter has never been directly detected, and nobody knows what it might be made of. A few scientists remeain skeptical. To a lay person, it might sound downright crazy. 

Now a new study suggests there may be no such thing as dark matter.

Fred Cooperstock of Northeastern University and Steven Tieu at the University of Victoria say Einstein's theory of general relativity can explain the cohesiveness of individual galaxies including our Milky Way. 

Here's the thinking: 

Newton's laws of physics explain why our solar system stays together. But the planets are negligible in the overall gravitational scheme, with the Sun being the total ruler and containing 99.86 percent of all the mass. 

The same Newtonian physics were long ago applied to galaxies, and the rotation of stars couldn't be explained, so dark matter was invented to make theory work.

But a galaxy is much different than the solar system, Cooperstock explains. The conglomeration of all the matter -- stars, black holes, gas, and dust -- is collectively the source of the galactic gravity. Even a black hole at a galaxy's center typically packs less than 1 percent of the galaxy's overall mass. 

The overall galaxy's gravity "feeds its own motion ... unlike the case of the solar system," Cooperstock told

The science of the new argument is complex, but here goes:

"In the galaxy case, having rotation, we have found that general relativity provides a very important potential that is connected to the density of the galactic matter in what we call a 'nonlinear' manner,'" Cooperstock says. "This is unlike Newtonian physics."

This nonlinear effect has been noted before. "The interesting twist is that this holds also for the simpler steady rotational motion under gravity as in the galaxy," he said.

The upshot: The motions of stars in galaxies "is realized in general relativity's equations without the need to invoke massive halos of exotic 'dark matter' that nobody can explain by current physics," Cooperstock said. 

A small percent of what used to be considered dark matter is made of burned-out stars that are hard to see. Predictions for how much of that material exists would not change. 

Also, the new idea does not yet explain how large clusters of galaxies bind together. Further research by other theorists might solve that problem too, however, Cooperstock said. The new analysis has been submitted to the Astrophysical Journal but has yet to be reviewed by other scientists. 

If it is right? 

"This would remove about 25 percent of the mass of the universe, the ultimate weight-reduction program," Cooperstock said.

Arip Nurahman
Guru dan Dosen Profesional

The End of the Dark Ages: First Light and Reionization

JWST Science: The End of the Dark Ages: First Light and Reionization

Edited by:
Arip Nurahman
(Teacher and Professional Lecturer)
Guru dan Dosen Profesional

Department of Physics, Faculty of Sciences and Mathematics
Indonesia University of Education and Follower Open Course Ware at MIT-Harvard University, M.A., U.S.A.

Until around 400 million years after the Big Bang, the Universe was a very dark place. There were no stars, and there were no galaxies. Scientists would like to unravel the story of exactly what happened after the Big Bang. The James Webb Space Telescope will pierce this veil of mystery and reveal the story of the formation of the first stars and galaxies in the Universe. 

hydrogen atom
As the Universe expanded after its origin in a Big Bang, the hot soup of fundamental particles (such as free protons and electrons) started to cool down. This allowed electrons and protons to pair up and form "neutral hydrogen atoms," (i.e. hydrogen atoms with one electron and one proton). This process of pairing up is called "Recombination" and it occurred about 400,000 years after the Big Bang. As the free electrons were now bound to protons, light could travel freely since it was no longer stopped by frequent scattering off the free electrons. 

The Universe went from being opaque to transparent at this point, and "the era of recombination" is the earliest point in our cosmic history to which we can look back with any form of light. This is what we see as the Cosmic Microwave Background today with satellites like the Cosmic Microwave Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe(WMAP). At right is an illustration of the timeline of the universe, courtesy of WMAP.
timeline of the universe

View a hi-res image
Another change occurred after the first stars formed. Theory predicts that the first stars were 30 to 300 times as massive as our Sun and millions of times as bright, burning for only a few million years before exploding as supernovae. The energetic ultraviolet light from these first stars was capable of splitting hydrogen atoms back into electrons and protons (or ionizing them). Observations of the spectra of distant quasars tell us that this occurred when the Universe was almost a billion years old.

That era, when the universe was a billion years old, is known as “the epoch of reionization.” It refers to the point when most of the neutral hydrogen was destroyed by the increasing radiation from the first massive stars. Reionization is an important phenomenon in our Universe’s history as it presents one of the few means by which we can (indirectly) study these earliest stars. But scientists do not know exactly when the first stars formed and when this reionization process started to occur. 

The Hubble Ultra Deep Field
The emergence of these first stars marks the end of the "Dark Ages" in cosmic history, a period characterized by the absence of discrete sources of light. Understanding these first sources is critical, since they greatly influenced the formation of later objects such as galaxies. The first sources of light act as seeds for the later formation of larger objects. (The Hubble Deep Field is shown at left.)

Additionally, the first stars that exploded as supernovae might have collapsed further to form black holes. The black holes started to swallow gas and other stars to become objects known as “mini-quasars,” which grew and merged to become the huge black holes now found at the centers of nearly all massive galaxies.
JWST will address several key questions to help us unravel the story of the formation of structures in the Universe such as: When and how did reionization occur?; What sources caused reionization?; What are the first galaxies? 

To find the first galaxies, JWST will make ultra-deep near-infrared surveys of the universe, and follow up with low-resolution spectroscopy and mid-infrared photometry (the measurement of the intensity of an astronomical object's electromagnetic radiation). To study reionization, high resolution near-infrared spectroscopy will be needed.

Arip Nurahman
Guru dan Dosen Profesional


Kondisi Cuaca Antariksa dan Orbit Satelit

Kondisi Cuaca Antariksa dan Orbit Satelit Info Aktivitas Matahari

Edited by:
Arip Nurahman
(Teacher and Professional Lecturer)
Guru dan Dosen Profesional

Department of Physics, Faculty of Sciences and Mathematics
Indonesia University of Education and Follower Open Course Ware at MIT-Harvard University, M.A., U.S.A.

Bilangan sunspot

Aktivitas matahari bervariasi dengan periode sekitar 11 tahun. Aktivitas matahari antara lain ditunjukkan oleh kemunculan bintik matahari (sunspot) di permukaannya. Umumnya bintik matahari muncul dalam satu kelompok (grup). Makin banyak bintik yang muncul di permukaan matahari, maka tingkat aktivitas matahari dikatakan makin tinggi, dan sebaliknya. Bilangan sunspot adalah parameter yang digunakan untuk menyatakan tingkat aktivitas matahari. Bilangan sunspot (R) dihitung sebagai berikut:

R = k(10g + f)

dengan k adalah faktor koreksi (tergantung pada pengamat dan peralatan)
  • g adalah banyaknya grup bintik yang muncul di permukaan matahari
  • f adalah banyaknya bintik individu
  • Data Bilangan Sunspot

  • Pengamatan Matahari
    Untuk mengetahui aktivitas matahari, perlu dilakukan pengamatan. Di LAPAN Bandung pengamatan dilakukan dengan menggunakan teleskop Celestron NextStar 8i, yang mempunyai diameter selebar 8 inchi dan menggunakan filter ND5 yang bisa mereduksi intensitas sinar matahari sebesar 105 kali. Pemotretan dilakukan dengan menggunakan kamera digital Nikon Coolpix 5400.
  • Data Pengamatan Matahari

  • Aktivitas matahari minggu ini
    Info parameter ionosfer Indonesia

    - Koreksi ionosfer model klobuchar
    Propagasi gelombang radio yang dipancarkan dari satelit ketika melewati ionosfer bumi akan mengalami perubahan akibat sifat medium dispersif ionosfer.
  • Data model klobuchar

  • - TEC (Total Electron Content)
    Propagasi gelombang radio melalui ionosfer akan mengalami delay time sebagai akibat dari keterkaitannya dengan electron bebas di ionosfer. Delay time ini dikarakteristikan oleh total electron content (TEC) ionosfer yang merupakan fungsi dari variable-variabel seperti lokasi geografis, waktu local, musim, radiasi eksrim UV (Ultra Violet) dan aktivitas medan magnet.
  • Data Pengamatan

  • - f0F2 dan hmF2
    Frekuensi kritis lapisan ionosfer (foF2) dan ketinggiannya (hmF2) adalah dua parameter lapisan ionosfer yang berkaitan dengan frekuensi kerja maksimum (Maximum Usable Frequency, MUF). Misalkan dua stasiun radio berjarak d kilometer dan dari peta diketahui foF2 dan hmF2 di titik tengahnya, maka MUF dapat dihitung menggunakan rumus pendekatan berikut :

    umus di atas digunakan dengan mengabaikan faktor kelengkungan permukaan bumi. engan melihat variasi f0F2 dan hmF2 secara spasial (terhadap lintang dan bujur) maupun temporal (terhadap waktu), maka dapat diketahui variasi MUF secara spasial dan temporal pula. Warna merah menunjukkan nilai kedua parameter tersebut lebih tinggi dan warna biru menujukkan nilai yang lebih rendah. selain itu, variasi (temporal) f0F2 jangka menengah dan panjang dapat digunakan sebagai indikasi respon lapisan ionosfer terhadap perubahan aktivitas matahari.


  • Peta f0F2 Indonesia
  • Peta hmF2 Indonesia

  • Info Aktivitas Geomagnet

    Variasi harian medan geomagnet merupakan fluktuasi medan magnet bumi setiap hari digunakan untuk memantau aktivitas medan magnet bumi dalam satuan nano tesla (nT) dan waktu universal (UT). Data Yang ditampilkan diambil dari stasion Pengamat Dirgantara Biak (BIK) terdiri atas tiga komponen H, D dan Z dimana,

  • Komponen H merupakan komponen medan magnet horizontal arah Utara - Selatan.
  • Komponen D merupakan komponen medan magnet horizontal arah Timur - Barat.
  • Komponen Z merupakan komponen medan magnet Vertikal

  • Arip Nurahman
    Guru dan Dosen Profesional