Wednesday, November 16, 2011
Bagaimana kita bisa mengatakan jika alam semesta lain telah bertabrakan dengan semesta yang kita tempati ? Fisikawan di Kanada dan Amerika Serikat percaya bahwa mereka memiliki jawabannya. Tabrakan itu akan meninggalkan jejak di Cosmic Microwave Background yang unik dan sangat berkarakter. Para ahli fisika mengklaim bahwa prediksi ini dapat diuji dengan menggunakan teleskop ruang angkasa yang ada saat ini ataupun teleskop masa depan, yang bertentangan dengan pandangan umum bahwa keberadaan multiverse telah teruji.
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| Cosmic Microwave Background |
Chuck Bennett, seorang astrofisikawan di Universitas Johns Hopkins di Maryland, AS, yang tidak terlibat dengan penelitian ini, mempercayai prediksi ini membantu membawa teori multiverse ke dalam relitas konvensional, pemalsuan ilmu (falsifiable science). "Ilmu bergantung pada kemampuan untuk memalsukan ide-ide melalui percobaan atau pengamatan alam," katanya. "Fakta bahwa ada potensi memungkinkan kita untuk memanggil Sains ini, bagi saya, adalah pernyataan yang signifikan.."
Kemungkinan multiverse datang dari teori string dan teori inflasi, sebuah gagasan bahwa alam semesta kita mengalami ekspansi yang cepat tepat setelah Big Bang. Teori Inflasi melakukan pekerjaan yang baik menjelaskan mengapa ruang cukup halus dalam skala besar, namun peneliti tidak dapat menjelaskan apa yang memulai ekspansi dan apa yang menghentikannya. Masalah-masalah ini telah menyebabkan fisikawan untuk mempertimbangkan kemungkinan bahwa inflasi dapat terjadi di ruang-waktu lain, menghasilkan alam semesta baru selain kita sendiri.
Kemungkinan multiverse datang dari teori string dan teori inflasi, sebuah gagasan bahwa alam semesta kita mengalami ekspansi yang cepat tepat setelah Big Bang. Teori Inflasi melakukan pekerjaan yang baik menjelaskan mengapa ruang cukup halus dalam skala besar, namun peneliti tidak dapat menjelaskan apa yang memulai ekspansi dan apa yang menghentikannya. Masalah-masalah ini telah menyebabkan fisikawan untuk mempertimbangkan kemungkinan bahwa inflasi dapat terjadi di ruang-waktu lain, menghasilkan alam semesta baru selain kita sendiri.
Masalah Metafisis
Ide tentang multiverse sangat kontroversial. Salah satu masalahnya adalah metafisika: alam semesta sudah nampak besar tanpa harus bersaing dengan sejumlah tak terbatas potensi semesta lain. Namun mungkin masalah yang lebih besar adalah ilmiah. Jika pengamatan terbatas pada alam "semesta teramati" kita sendiri, bagaimana bisa para ilmuwan menguji "apakah ada multiverse yang lebih besar?". Jawabannya adalah bahwa, dari waktu ke waktu, alam semesta lain di multiverse mungkin berbenturan dengan semesta kita, meninggalkan sebuah alur di jejaknya. Tapi mencari tahu secara pasti bagaimana bentuk alur akan terlihat tidaklah mudah.
Now, however, Kris Sigurdson of the University of British Columbia in Vancouver and others say they have calculated the detailed features of a cosmic wake. They have considered the possibility that our universe collided with another before our inflation period, because, they say, the latter would have erased the wake's evidence. Even though this happened more than 13 billion years ago, the wake would have been preserved in the cosmic microwave background (CMB), which was formed some 380,000 years into the universe's existence.
Look for a 'double peak'
The focus of the prediction is in the polarization of photons in the CMB. Photons have two transverse polarization states, and any that come from a certain region in the CMB might be mostly in the same polarization state, or in a mix of both. Sigurdson and colleagues calculate that, providing the wake was big enough, it ought to imprint the CMB with a characteristic "double peak": two close rings where the photons sway towards a single polarization state.
The prediction is not strictly the first to arise from multiverse theory. In 2007 researchers at the University of California at Santa Cruz, US, also suggested that a cosmic wake could imprint itself on the CMB; then, earlier this year, a group led by Hiranya Peiris of University College London found hints that this prediction was true. But these predicted features were too vague, say Sigurdson and colleagues, and might have existed in the CMB anyway.
Evidence for string theory?
"[Our] features represent the first verifiable prediction of the multiverse paradigm," write Sigurdson and colleagues in their preprint, which they uploaded to the arXiv server last month. "A detection of a bubble collision would confirm the existence of the multiverse, provide compelling evidence for the string theory landscape, and sharpen out picture of the universe and its origins." Physics World was unable to speak to the researchers about their preprint because they are submitting it to a journal that employs an embargo policy.
If the prediction is correct, it should be possible to test it in upcoming data from the European Space Agency's Planck space observatory and future CMB missions, say the researchers. Yet Bennett, the principal investigator on NASA's Wilkinson Microwave Anisotropy Probe, another CMB space observatory, thinks the detection of a cosmic wake would nonetheless be "extremely unlikely". He says the amplitude of a wake would have to be just right: too small and we wouldn't see it; too big and it would probably have had severe consequences for our universe's structure. The number of collisions would also have to be "fine-tuned", he says.
Infinite number of wakes
"The claim seems to be that we might see one or two wakes in our sky, but why one or two?" he adds. "Why not none or an infinite number? In fact, if bubble collisions were common we would not be alive to discuss the question."
Cosmologist Arjun Berera at the University of Edinburgh, UK, also thinks the idea of a multiverse – and by extension Sigurdson and colleagues' prediction – is speculative. But he notes that a positive detection would be "spectacular". "Such a case would offer suggestive evidence in support of string theory," he says. "On the other hand, no evidence in the CMB data for a collision between two universes would not rule out string theory, it would simply extend the widely held belief in the field that string theory is unfalsifiable."
The research is described in arXiv:1109.3473.
