{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T02:47:47Z","timestamp":1760150867845,"version":"build-2065373602"},"reference-count":21,"publisher":"MDPI AG","issue":"2","license":[{"start":{"date-parts":[[2022,1,31]],"date-time":"2022-01-31T00:00:00Z","timestamp":1643587200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Symmetry"],"abstract":"<jats:p>According to general relativity (GR), a universe with a cosmological constant \u039b, like ours, is trapped inside an event horizon, r&lt;3\/\u039b. What is outside? We show, using Israel (1967) junction conditions, that there could be a different universe outside. Our universe looks like a black hole for an outside observer. Outgoing radial null geodesics cannot escape our universe, but incoming photons can enter and leave an imprint on our CMB sky. We present a picture of such a fossil record from the analysis of CMB maps that agrees with the black hole universe predictions but challenges our understanding of the origin of the primordial universe.<\/jats:p>","DOI":"10.3390\/sym14020285","type":"journal-article","created":{"date-parts":[[2022,1,31]],"date-time":"2022-01-31T01:46:21Z","timestamp":1643593581000},"page":"285","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":7,"title":["A Peek Outside Our Universe"],"prefix":"10.3390","volume":"14","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-9632-0815","authenticated-orcid":false,"given":"Enrique","family":"Gaztanaga","sequence":"first","affiliation":[{"name":"Institute of Space Sciences (ICE, CSIC), 08193 Barcelona, Spain"},{"name":"Institut d Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1510-5214","authenticated-orcid":false,"given":"Pablo","family":"Fosalba","sequence":"additional","affiliation":[{"name":"Institute of Space Sciences (ICE, CSIC), 08193 Barcelona, Spain"},{"name":"Institut d Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain"}]}],"member":"1968","published-online":{"date-parts":[[2022,1,31]]},"reference":[{"doi-asserted-by":"crossref","unstructured":"Padmanabhan, T. 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A new perspective on steady-state cosmology. arXiv.","key":"ref_5"},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"436","DOI":"10.1093\/mnras\/stab056","article-title":"The cosmological constant as a zero action boundary","volume":"502","year":"2021","journal-title":"Mon. Not. R. Astron. Soc."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"2766","DOI":"10.1093\/mnras\/staa1000","article-title":"The size of our causal Universe","volume":"494","author":"Gaztanaga","year":"2020","journal-title":"Mon. Not. R. Astron. Soc."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"104044","DOI":"10.1103\/PhysRevD.81.104044","article-title":"McVittie\u2019s legacy: Black holes in an expanding universe","volume":"81","author":"Kaloper","year":"2010","journal-title":"Phys. Rev. D"},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"463","DOI":"10.1007\/BF02712210","article-title":"Singular hypersurfaces and thin shells in general relativity","volume":"48","author":"Israel","year":"1967","journal-title":"Nuovo Cimento B Serie"},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"788","DOI":"10.1119\/1.17460","article-title":"The observable universe inside a black hole","volume":"62","author":"Stuckey","year":"1994","journal-title":"Am. J. Phys."},{"key":"ref_11","first-page":"682","article-title":"Spectrum of relict gravitational radiation and the early state of the universe","volume":"30","year":"1979","journal-title":"Sov. Exp. Theor. Phys. Lett."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"347","DOI":"10.1103\/PhysRevD.23.347","article-title":"Inflationary universe: A possible solution to the horizon and flatness problems","volume":"23","author":"Guth","year":"1981","journal-title":"Phys. Rev. D"},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"389","DOI":"10.1016\/0370-2693(82)91219-9","article-title":"A new inflationary universe scenario: A possible solution of the horizon, flatness, homogeneity, isotropy and primordial monopole problems","volume":"108","author":"Linde","year":"1982","journal-title":"Phys. Lett. B"},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"1220","DOI":"10.1103\/PhysRevLett.48.1220","article-title":"Cosmology for Grand Unified Theories with Radiatively Induced Symmetry Breaking","volume":"48","author":"Albrecht","year":"1982","journal-title":"Phys. Rev. Lett."},{"doi-asserted-by":"crossref","unstructured":"Planck Collaboration (2020). Planck 2018 results. VII. Isotropy and statistics of the CMB. Astron. Astrophys., 641, A7.","key":"ref_15","DOI":"10.1051\/0004-6361\/201832909"},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"184001","DOI":"10.1088\/0264-9381\/33\/18\/184001","article-title":"CMB anomalies after Planck","volume":"33","author":"Schwarz","year":"2016","journal-title":"Class. Quantum Gravity"},{"doi-asserted-by":"crossref","unstructured":"Planck Collaboration (2020). Planck 2018 results. VI. Cosmological parameters. Astron. Astrophys., 641, A6.","key":"ref_17","DOI":"10.1051\/0004-6361\/201832909"},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"10","DOI":"10.1038\/s42254-019-0137-0","article-title":"The expansion of the Universe is faster than expected","volume":"2","author":"Riess","year":"2019","journal-title":"Nat. Rev. Phys."},{"unstructured":"DES Collaboration (2019). Cosmological Constraints from Multiple Probes in the Dark Energy Survey. Phys. Rev. Lett., 122, 171301.","key":"ref_19"},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"153001","DOI":"10.1088\/1361-6382\/ac086d","article-title":"In the realm of the Hubble tension-a review of solutions","volume":"38","author":"Mena","year":"2021","journal-title":"Class. Quantum Gravity"},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"5840","DOI":"10.1093\/mnras\/stab1193","article-title":"Explaining cosmological anisotropy: Evidence for causal horizons from CMB data","volume":"504","author":"Fosalba","year":"2021","journal-title":"Mon. Not. R. Astron. 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